Add explicit uint16_t cast in aom_memset16()

Add an explicit uint16_t cast in aom_memset16() to avoid
implicit signed-to-unsigned conversions when it is called
from the fill_residue_outside_frame().

Bug: 527078408, 527242002

Change-Id: I33b302a1bd56c6857e5523cc5dda8bb105a29de3

aom_mem/aom_mem.h

diff --git a/aom_mem/aom_mem.h b/aom_mem/aom_mem.h
index b332139..27e102e 100644
--- a/aom_mem/aom_mem.h
+++ b/aom_mem/aom_mem.h
@@ -42,7 +42,7 @@
 static inline void *aom_memset16(void *dest, int val, size_t length) {
   size_t i;
   uint16_t *dest16 = (uint16_t *)dest;
-  for (i = 0; i < length; i++) *dest16++ = val;
+  for (i = 0; i < length; i++) *dest16++ = (uint16_t)val;
   return dest;
 }