Properly upgrade int before operation

Change-Id: Ibca63a41bdffaab2ee603497e8a5fc66d5b7d11f

aom_ports/aom_timer.h

@@ -82,7 +82,7 @@ static INLINE int64_t aom_usec_timer_elapsed(struct aom_usec_timer *t) {
   struct timeval diff;
 
   timersub(&t->end, &t->begin, &diff);
-  return (int64_t)diff.tv_sec * 1000000 + diff.tv_usec;
+  return ((int64_t)diff.tv_sec) * 1000000 + diff.tv_usec;
 #endif
 }