When a packer instance has a dict as its ext handler and it is passed None, it segfaults after printing Exception TypeError: "'NoneType' object has no attribute '__getitem__'" in 'mpack.unparse_enter' ignored.
The problem doesn't appear when the handler is something else, nor with anything but None, nor when unpacking an ext.

Travis proof: https://travis-ci.org/lourkeur/libmpack-python/builds/229769196https://travis-ci.org/lourkeur/libmpack-python/builds/229764123