Segfault when packing and ext handler is a dict

[bbjubjub2494]

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

[bbjubjub2494]

Correction: it doesn't need to be None, it can't be anything that's not handled by the normal mechanism and thus goes to the handler. The fact that it goes off on None is due to #1.