/ext/standard: Check for empty string in linkinfo()

[LamentXU123]

If link_len == 0, it now emits the same warning style as the existing filesystem error path and returns -1 immediately, avoiding the later estrndup() / zend_dirname() work on an empty input as per the TODO message.

[LamentXU123]

Ideally we'd be able to audit all the call sites and basically just mandate that the path passed to zend_dirname() must not be empty nor contain null bytes.

AFAIK This can be fixed inside the zend_dirname function. A simple check for the arguments would work (?) We may still need to customize the error message by passing some info (i.e. php function name that use zend_dirname) to the API to make the error message precise.

[Girgias]

Ideally we'd be able to audit all the call sites and basically just mandate that the path passed to zend_dirname() must not be empty nor contain null bytes.

AFAIK This can be fixed inside the zend_dirname function. A simple check for the arguments would work (?) We may still need to customize the error message by passing some info (i.e. php function name that use zend_dirname) to the API to make the error message precise.

I don't want to pass the argnum to a Zend API, it is better to enforce the semantics at the call site as I can't imagine most use sites of this API have empty strings.

[Girgias]

UPGRADING also needs to be updated.