The fast-path unmarshal was getting confused when parsing a group
containing a field with a number the same as the group's own field
number. Separate the handling of EndGroup tags.
Change-Id: I637702b42c94a26102e693ee29a55e80b37d7f28
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/214737
Reviewed-by: Joe Tsai <joetsai@google.com>