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>
