vp8/encoder/boolhuff: fix integer sanitizer warnings

from sanitizer run:
runtime error: implicit conversion from type 'unsigned int' of value 256
(32-bit, unsigned) to type 'unsigned char' changed the value to
 0 (8-bit, unsigned)

BUG=webm:1615

Change-Id: I9321bbd58a305419bc8669ecd7594adc47e8b116
1 file changed