fix mutex unlock error on callback panic (#49)

[nbaztec]

This is a bugfix for #49

The code would try to exit following a panic in a callback and would rightly attempt to RUnlock the mutex since it's on the defer stack, however, the matching RLock call never gets called.

The fix puts the matching RLock call also on the defer stack in the event that a panic occurs.

[maxekman]

LGTM. Thanks for fixing the issue.