warped_motion: Fix ubsan warning for signed integer overflow

Change-Id: Ie698aa02ef56128759c71079e9bfa1af25149644

av1/common/warped_motion.c

@@ -215,7 +215,7 @@ static int32_t do_cubic_filter(int32_t *p, int x) {
   } else if (x == (1 << WARPEDPIXEL_PREC_BITS)) {
     return p[1];
   } else {
-    const int64_t v1 = x * x * x * (3 * (p[0] - p[1]) + p[2] - p[-1]);
+    const int64_t v1 = (int64_t)x * x * x * (3 * (p[0] - p[1]) + p[2] - p[-1]);
     const int64_t v2 = x * x * (2 * p[-1] - 5 * p[0] + 4 * p[1] - p[2]);
     const int64_t v3 = x * (p[1] - p[-1]);
     const int64_t v4 = 2 * p[0];