onscreen/native: Warn instead of crashing if the frame_info queue is empty
Some Ubuntu systems with mutter 40 and 41 report maybe_update_frame_info
crashes with SIGSEGV when frame_info == NULL
.
The same was reported by @rmader as happening "after wakeup" in testing (!1441 (comment 1301588)) although the Ubuntu reports show it's not related to that MR.