Fix shift of signed value in unsigned bit reader.
Fix shift of signed value in unsigned bit reader.

BUG=aomedia:1723

Change-Id: I3f14fe55507f464046c23bd4d10082a03a36772d

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