Re-allocate the mfmv buffer when frame size changes
Re-allocate the mfmv buffer when frame size changes

Fix a bug that prevents the mfmv buffer re-allocation when the
frame size changes.

BUG=aomedia:1194

Change-Id: Ied349ead9703069c87fbf9e46668ae1dd0b6141a

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