av1/encoder/x86/pickrst_sse4: fix int sanitizer warnings

implicit conversion from type 'int' of value -784 (32-bit, signed) to
type 'uint16_t' (aka 'unsigned short') changed the value to 64752
(16-bit, unsigned)

BUG=aomedia:2357

Change-Id: Ic78f6f0379105a673f419458ec937bf29560c761

av1/encoder/x86/pickrst_sse4.c

diff --git a/av1/encoder/x86/pickrst_sse4.c b/av1/encoder/x86/pickrst_sse4.c
index f41694b..a2f65a5 100644
--- a/av1/encoder/x86/pickrst_sse4.c
+++ b/av1/encoder/x86/pickrst_sse4.c
@@ -497,7 +497,7 @@
   }
 }
 
-static INLINE __m128i pair_set_epi16(uint16_t a, uint16_t b) {
+static INLINE __m128i pair_set_epi16(int a, int b) {
   return _mm_set1_epi32((int32_t)(((uint16_t)(a)) | (((uint32_t)(b)) << 16)));
 }