file: do not attempt to open(NULL, ...); (fix #6796)

(cherry picked from commit 9984e74dbff847645ae0e63db96bcd249568b7a4)

modules/access/file.c

@@ -172,6 +172,8 @@ int FileOpen( vlc_object_t *p_this )
     {
         const char *path = p_access->psz_filepath;
 
+        if (unlikely(path == NULL))
+            return VLC_EGENERIC;
         msg_Dbg (p_access, "opening file `%s'", path);
         fd = vlc_open (path, O_RDONLY | O_NONBLOCK);
         if (fd == -1)