The error-resilient-mode flag is currently set per frame,
so we can encode a non-error-resilient frame immediately
after an error-resilient frame.

As of https://aomedia-review.googlesource.com/c/21783 , this
case causes the decoder to read uninitialized memory. This
happens because the error-resilient frame no longer sets up
cm->cur_frame->global_motion. Then, on the non-error-resilient
frame, that is renamed to cm->prev_frame->global_motion and we
try to use it as a reference for delta encoding.

Fix this by always writing cm->cur_frame->global_motion.

Change-Id: I0d4f13bf413ca15da210e06d5ce1780331c005da