av1/encoder/x86/av1_quantize_sse2: fix int sanitizer warnings

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=aomedia:2357

Change-Id: I4ffdd66beaf041cc97979ab2230e186469aa92e6
diff --git a/av1/encoder/x86/av1_quantize_sse2.c b/av1/encoder/x86/av1_quantize_sse2.c
index b07e771..5497c7e 100644
--- a/av1/encoder/x86/av1_quantize_sse2.c
+++ b/av1/encoder/x86/av1_quantize_sse2.c
@@ -91,7 +91,7 @@
                                      _mm_cmpeq_epi16(qcoeff0, *thr0));
   const __m128i mask1 = _mm_or_si128(_mm_cmpgt_epi16(qcoeff1, *thr1),
                                      _mm_cmpeq_epi16(qcoeff1, *thr1));
-  const int16_t nzflag = _mm_movemask_epi8(mask0) | _mm_movemask_epi8(mask1);
+  const int nzflag = _mm_movemask_epi8(mask0) | _mm_movemask_epi8(mask1);
 
   if (nzflag) {
     qcoeff0 = _mm_adds_epi16(qcoeff0, *round0);