vc1dec: fix a possible unitialized variable warning

libavcodec/vc1dec.c

@@ -1868,7 +1868,8 @@ static inline void vc1_pred_mv_intfr(VC1Context *v, int n, int dmv_x, int dmv_y,
                 } else if (c_valid) {
                     px = C[0];
                     py = C[1];
-                }
+                } else
+                    px = py = 0;
             }
         } else if (total_valid == 1) {
             px = (a_valid) ? A[0] : ((b_valid) ? B[0] : C[0]);