[php-src] PR #22954: Fix mbregex search state after cache invalidation

Pull Request: https://github.com/php/php-src/pull/22954 Author: matthiasgoergens Fixes GH-21036. php_mbregex_compile_pattern() may replace the cache entry currently referenced by search_re. The replacement destroys that regex and clears search_re, but leaves search_regs holding match offsets associated with the destroyed regex. mb_ereg_search_getregs() then treats those registers as valid and dereferences the null regex when processing named captures. Free and clear search_regs whenever cache replacement clears search_re. This preserves the existing invariant that match registers do not outlive their regex and makes mb_ereg_search_getregs() return false instead of returning stale numeric captures while losing named captures. PR #21038 identified the crash and proposed guarding the named-capture lookup. This alternative also discards the invalidated match state, following the approach used for the analogous invalid-pattern path in commit 392ad206a4f63fedf61d8086e390c73de8b72767.