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

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

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Co-authored-by: hourhl <redacted>
Co-authored-by: Sigbjørn Skjæret <redacted>