UPSTREAM PR #20445: tests: Fix invalid iterator::end() dereference in common_regex

[loci-dev]

Note

Source pull request: ggml-org/llama.cpp#20445

When compiling with VS2026 18.4 I noticed test-regex-partial crashes immediately with debug build.

I tracked this down to an iterator::end() dereference in the following test case which was occurring here.

    assert_equals<std::string>(
        "^(a*)",
        regex_to_reversed_partial_regex("a*"));

This fix adds bounds checking to prevent such dereferencing.

[loci-review]

No meaningful performance changes were detected across 120084 analyzed functions in the following binaries: build.bin.libllama.so, build.bin.llama-tts, build.bin.llama-cvector-generator, build.bin.libmtmd.so, build.bin.llama-bench, build.bin.libggml-base.so, build.bin.libggml-cpu.so, build.bin.libggml.so, build.bin.llama-quantize, build.bin.llama-qwen2vl-cli, build.bin.llama-tokenize, build.bin.llama-gemma3-cli, build.bin.llama-gguf-split, build.bin.llama-llava-cli, build.bin.llama-minicpmv-cli.

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