search-directory-file: Check for NULL before calling callback
Prevent segfault when calling call_when_ready with a NULL callback
This is the case with sidebar row, dc2711bd which attempts to get the mount information from a file. vfs-file chains up to nautilus-directory and handles the NULL callback fine, but if somehow, the search-directory was added to the bookmarks (which isn't possible in current versions of Nautilus, but maybe was in past version), Nautilus crashes.
Fixes: #2513 (closed)