Fixing a crash in set_multi with unicode keys

When set_multi is called with multiple keys and the keys are
not string (aka they do not serialize well) the code tries
to clean up datastructures that were not initialized.
This causes segmentation fault. Basically we should only
clean up the objects we have allocated.

fix for issue #116
Merge pull request #117 from NikolaBorisov/fix-set-multi-crash

Fixing a crash in set_multi with unicode keys