vpx_dsp/bitwriter.h: fix clang integer sanitizer warning

implicit conversion from type 'unsigned int' of value 256 (32-bit, unsigned)
to type 'uint8_t' (aka 'unsigned char') changed the value to 0 (8-bit, unsigned)


BUG=webm:1615

Change-Id: Ia9ac3772021ae492368c650a73846e7d22c8fdfc
1 file changed