Replace by CAST_TO_BYTEPTR/SHORTPTR.
The rule is: if a short ptr is casted to a byte ptr, any offset
operation on the byte ptr must be doubled. We do this by casting to
short ptr first, adding offset, then casting back to byte ptr.

BUG=webm:1388

Change-Id: I9e18a73ba45ddae58fc9dae470c0ff34951fe248