CajaFile: fix crash in modify_link_hash_table

We crash in g_hash_table_lookup_extended if the file's symlink_name is
set but the file is not a symlink, since the hash function (g_str_hash)
does not allow NULL values but target_uri is NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=711583
Adapted from https://gitlab.gnome.org/GNOME/nautilus/commit/c2a5e052

libcaja-private/caja-file.c

@@ -555,7 +555,7 @@ modify_link_hash_table (CajaFile *file,
 	GList **list_ptr;
 
 	/* Check if there is a symlink name. If none, we are OK. */
-	if (file->details->symlink_name == NULL) {
+	if (file->details->symlink_name == NULL || !caja_file_is_symbolic_link (file)) {
 		return;
 	}