internal/impl: fix validation of required group fields

Change-Id: I3c3b5cfbea599dc08096aa5992b7829c2e50f25d
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/218578
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/impl/validate.go b/internal/impl/validate.go
index e568c75..06acc78 100644
--- a/internal/impl/validate.go
+++ b/internal/impl/validate.go
@@ -343,8 +343,10 @@
 					ok = wtyp == wire.Fixed32Type
 				case validationTypeFixed64:
 					ok = wtyp == wire.Fixed64Type
-				case validationTypeBytes, validationTypeUTF8String, validationTypeMessage, validationTypeGroup:
+				case validationTypeBytes, validationTypeUTF8String, validationTypeMessage:
 					ok = wtyp == wire.BytesType
+				case validationTypeGroup:
+					ok = wtyp == wire.StartGroupType
 				}
 				if ok {
 					st.requiredMask |= vi.requiredBit