Initialise ext buffer after decoding keyframes
Initialise ext buffer after decoding keyframes

Prior to this patch, buffers for all reference frames except for the
ext buffer were initialised after decoding a keyframe.

This patch includes the ext buffer in this initialisation.

Change-Id: Ia3e1fe38553ea57ba9b279b75b9dd45afd6ab42d

GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom,GrokImageCompression/aom