Dynomite may crash in req_forward_error() if msg_get_error() returns NULL

[smukil]

https://github.com/Netflix/dynomite/blob/dev/src/dyn_client.c#L420-L421

msg_get_error() may return a NULL pointer for a few reasons. https://github.com/Netflix/dynomite/blob/df884c0dde610d2e85aed2b9f18bcf8d7ebebd7b/src/dyn_message.c#L512-L550

If this happens, we would dereference a NULL pointer leading to a crash. I still don't have a clean fix in mind since the req_forward_error() function return type is 'void', so figuring out what action to take in this case seems non-trivial. Once I find an approach, I can submit a patch.