integration: check Watch response error not nil to avoid runtime panic.

Fixes issue: #14259

Signed-off-by: 王霄霄 <1141195807@qq.com>

tests/integration/clientv3/watch_test.go

@@ -340,6 +340,9 @@ func putAndWatch(t *testing.T, wctx *watchctx, key, val string) {
 		if !ok {
 			t.Fatalf("unexpected watch close")
 		}
+		if err := v.Err(); err != nil {
+			t.Fatalf("unexpected watch response error: %v", err)
+		}
 		if string(v.Events[0].Kv.Value) != val {
 			t.Fatalf("bad value got %v, wanted %v", v.Events[0].Kv.Value, val)
 		}