Memory allocated for outbound RDP clipboard data is not properly freed

As reported on the mailing list, the memory allocated for converting received clipboard data is not freed, resulting in a memory leak when data is copied locally:

https://lists.apache.org/thread.html/r9b32375ba4172ee374619f827540a358794ffea89cb6800083de924c%40%3Cdev.guacamole.apache.org%3E

The relevant memory should be freed when no longer in use.