When creating a MessageDescriptor for a legacy message with a
Descriptor method, we call that method on the type's zero value to
get the message's DescriptorProto. Some existing dynamic message
types have a Descriptor method which panics in this case.
Catch the panic and continue as if the Descriptor method wasn't present.
Change-Id: I98d4625d6917cc1ec25737e5670a443f5d02a404
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/206637
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
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