Fix #73025: Heap Buffer Overflow in virtual_popen of zend_virtual_cwd.c

Fix #73025: Heap Buffer Overflow in virtual_popen of zend_virtual_cwd.c

`command_length` is retrieved via strlen() and later passed to emalloc()
and memcpy(), so the appropriate type is `size_t`.

We don't add a regression test, because that would need to allocate a string
of at least 2 GiB.