fix: PhotoMaker: avoid GGML_ASSERT if trigger word was not found in prompt

[akleine]

Summary

Currently, a missing trigger word ("img") in PhotoMaker's prompt is not handled correctly (see below).
I added an additional check for class_idx >= 0 when creating the class_token_mask.

That leads to this message:
[WARN ] photomaker_extension.cpp:232 - Turn off PhotoMaker for this request
and continuation of the program just like in similar situations (e.g. "Photomaker ID Stacking failed" etc.) .

Related Issue / Discussion

Note: class_idx stays at initial values of -1 if we do not get the trigger word in the loop before.

Additional Information

Before this little patch:

[DEBUG] bpe_tokenizer.cpp:208  - split prompt "img" to tokens ["img</w>", ]                                                                                                                   
[DEBUG] conditioner.hpp:347  - parse 'man is playing a guitar' to [['man is playing a guitar', 1], ]                                                                                          
[DEBUG] bpe_tokenizer.cpp:208  - split prompt "man is playing a guitar" to tokens ["man</w>", "is</w>", "playing</w>", "a</w>", "guitar</w>", ]                                               
/home/usr/stable-diffusion.cpp/src/extensions/photomaker_extension.cpp:99: GGML_ASSERT(it != tokens.end()) failed                                                                             
. . .                                                                                                                                                                                           
#0  0x0000713df0910813 in __GI___wait4 (pid=4095, stat_loc=0x0, options=0, usage=0x0) at ../sysdeps/unix/sysv/linux/wait4.c:30                                                                
#1  0x0000556b6c5dace3 in ggml_print_backtrace ()                                                                                                                                             
#2  0x0000556b6c5dae87 in ggml_abort ()                                                                                                                                                       
#3  0x0000556b6b87d00c in PhotoMakerExtension::prepare_condition(GenerationExtensionConditionContext&) ()                                                                                     
#4  0x0000556b6b6117cf in prepare_image_generation_embeds(sd_ctx_t*, sd_img_gen_params_t const*, GenerationRequest*, SamplePlan*, ImageGenerationLatents*, RefImageParams const&) ()          
#5  0x0000556b6b61a706 in generate_image ()                                                                                                                                                   
#6  0x0000556b6b404130 in main ()                                                                                                                                                             
[Inferior 1 (process 3985) detached]                                                                                                                                                          
./0_NEWTON_MAN_FRU: Zeile 11:  3985 Abort (core dumped) ./sd-cli -m ~/SD_models/sdxl/zavychromaxl_v100.safetensors --vae ~/SD_models/sdxl/sdxl_vae.safetensors --clip-on-cpu --steps 20 -v --s

Now we have:

[DEBUG] bpe_tokenizer.cpp:208  - split prompt "img" to tokens ["img</w>", ]                                                                                                                   
[DEBUG] conditioner.hpp:347  - parse 'man is playing a guitar' to [['man is playing a guitar', 1], ]                                                                                          
[DEBUG] bpe_tokenizer.cpp:208  - split prompt "man is playing a guitar" to tokens ["man</w>", "is</w>", "playing</w>", "a</w>", "guitar</w>", ]                                               
....                                                                                                                                                                                          
[WARN ] photomaker_extension.cpp:231  - PhotoMaker trigger word 'img' was not found in prompt                                                                                                 
[WARN ] photomaker_extension.cpp:232  - Turn off PhotoMaker for this request                                                                                                                  
[DEBUG] conditioner.hpp:347  - parse 'man is playing a guitar' to [['man is playing a guitar', 1], ]                                                                                          
[DEBUG] bpe_tokenizer.cpp:208  - split prompt "man is playing a guitar" to tokens ["man</w>", "is</w>", "playing</w>", "a</w>", "guitar</w>", ]                                               
[DEBUG] ggml_extend.hpp:2175 - clip compute buffer size: 1.42 MB(RAM)                                                                                                                         . . .

Checklist

• I have read and confirmed this PR follows the contribution guidelines.