Save bits by not encoding global motion data when there's no
previous frame to use as a reference. Use the same logic
as for using the previous frame's motion vectors - ie, we don't
read global motion information when the current frame is
intra-only or when using error-resilient mode.

This also fixes an undefined behaviour / segfault bug - see
https://bugs.chromium.org/p/aomedia/issues/detail?id=731 for
details

BUG=aomedia:731

Change-Id: Icca90a1bccafd06de8a4056ca5353318fce416cb