encoder/x86/pickrst_avx2: fix int sanitizer warnings

implicit conversion from type 'int' of value -71 (32-bit,
signed) to type 'uint16_t' (aka 'unsigned short') changed the value to
65465 (16-bit, unsigned)

BUG=aomedia:2357

Change-Id: I0f0c3b378007155f3cd470b48e71eb028bd557b9
diff --git a/av1/encoder/x86/pickrst_avx2.c b/av1/encoder/x86/pickrst_avx2.c
index e011c26..aa7b7c1 100644
--- a/av1/encoder/x86/pickrst_avx2.c
+++ b/av1/encoder/x86/pickrst_avx2.c
@@ -493,7 +493,7 @@
   }
 }
 
-static INLINE __m256i pair_set_epi16(uint16_t a, uint16_t b) {
+static INLINE __m256i pair_set_epi16(int a, int b) {
   return _mm256_set1_epi32(
       (int32_t)(((uint16_t)(a)) | (((uint32_t)(b)) << 16)));
 }