vd_lavc: fix return value in receive_frame()

Pointed out by uau. Not sure why gcc doesn't warn (it uses ((void*)0)
for NULL).

video/decode/vd_lavc.c

@@ -1133,7 +1133,7 @@ static bool receive_frame(struct dec_video *vd, struct mp_image **out_image)
             MP_ERR(vd, "Could not copy back hardware decoded frame.\n");
             ctx->hwdec_fail_count = INT_MAX - 1; // force fallback
             handle_err(vd);
-            return NULL;
+            return false;
         }
     }