internal/impl: recognized required bytes fields in validation

Add a missed case in validation so we correctly validate bytes fields.
Fixes a case where we would report required bytes fields as potentially
missing.

Change-Id: I3dc4196d6995942d32a795a64214b3679d60ab6c
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/215000
Reviewed-by: Joe Tsai <joetsai@google.com>
diff --git a/internal/impl/validate.go b/internal/impl/validate.go
index 5f3c3f5..1d34063 100644
--- a/internal/impl/validate.go
+++ b/internal/impl/validate.go
@@ -208,6 +208,8 @@
 				vi.typ = validationTypeFixed32
 			case wire.Fixed64Type:
 				vi.typ = validationTypeFixed64
+			case wire.BytesType:
+				vi.typ = validationTypeBytes
 			}
 		}
 	}