Fix identity handling for inproc transport

Fixes #872

Backport of hurtonm/libzmq@a09407829; included as
zeromq/libzmq@e376ad278 in development repos.
Merge pull request #79 from olafmandel/master

Fix identity handling for inproc transport
Merge branch 'master' of https://github.com/zeromq/zeromq4-x