b9917: fix: OOB reads in UGM tokenizer (precompiled_charsmap handling) (#18750)

• fix: OOB reads in UGM tokenizer (precompiled_charsmap handling)

• Validate minimum size (4 bytes) before reading xcda_blob_size
• Use strnlen with bounds check instead of unsafe strlen

Both issues allow heap-buffer-overflow from malicious T5/UGM GGUF files.

• Replace unsafe strnlen() with a bounds-checked loop that scans for \0 within the remaining array size.

• move bounds checks to load

• typo merge fix

Co-authored-by: hourhl [email protected]
Co-authored-by: Sigbjørn Skjæret [email protected]