vp9_quantize_sse2: quiet clang-7 integer sanitizer warning

nzflag is used as a boolean, it doesn't need to be a sized type, int is
enough (and _mm_movemask_epi8 returns one)

fixes:
vp9_quantize_sse2.c:136:16: implicit conversion from type
'int' of value 65535 (32-bit, signed) to type 'int16_t' (aka 'short')
changed the value to -1 (16-bit, signed)

BUG=webm:1649

Change-Id: I0e3f5278af49d84760f3dfb607f28099cf02f21d
1 file changed