tests: Fix invalid iterator::end() dereference in common_regex

[rillomas]

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.

[rillomas]

Hi @ggerganov, @CISC can you have a look at this (or assign someone sufficient)? Thanks

[aldehir]

I believe we have eliminated most, if not all, uses of the partial regex implementation.

Still, I reproduced this and the fix works. Thanks!