sapi/phpdbg: restore the DEFAULT_SLASH check on the module path

[darkdi]

Before GH-21578 this guard read strchr(*path, '/') || strchr(*path, DEFAULT_SLASH). The rewrite onto memchr() kept the first half and copied it into the second, so both now look for '/', and the DEFAULT_SLASH half is gone. On Windows that means ext C:\path\php_foo.dll is not treated as a path at all, extension_dir gets glued in front of it and the load fails. ext/standard/dl.c still has the pair intact, which is what the phpdbg code was written from. Only master carries this, 8.4 and 8.5 still have the strchr version, and I have not touched the NEWS file since nothing released is affected.

Disclosure per CONTRIBUTING: the paragraph above was drafted with an LLM (Claude Code). The finding came from a static check of my own and I read the code and the history by hand.

🤖 Generated with Claude Code