encoder/temporal_filter.c: fix ubsan warnings

Change-Id: I30fa12202f29524984647d97ca363cb40bc68208

av1/encoder/temporal_filter.c

@@ -100,8 +100,8 @@ void av1_temporal_filter_apply_c(uint8_t *frame1, unsigned int stride,
 
       for (idy = -1; idy <= 1; ++idy) {
         for (idx = -1; idx <= 1; ++idx) {
-          int row = i + idy;
-          int col = j + idx;
+          int row = (int)i + idy;
+          int col = (int)j + idx;
 
           if (row >= 0 && row < (int)block_height && col >= 0 &&
               col < (int)block_width) {