Fix crash in AMVR (take 2)
Fix crash in AMVR (take 2)

Do not call is_integer_mv() unless the previous frame is
valid in the lookahead buffer.

BUG=aomedia:1466

Change-Id: I435e211298c2a89d877eeb605cd6cb0dd6b811f0

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