Remove redundant tor_free() in command_process_create_cell().
* FIXES #16823: https://bugs.torproject.org/16823
If an OP were to send a CREATE_FAST cell to an OR, and that
CREATE_FAST cell had unparseable key material, then tor_free() would
be called on the create cell twice. This fix removes the second
(conditional on the key material being bad) call to tor_free(), so
that now the create cell is always freed once, regardless of the status of
the key material.
(This isn't actually a double-free bug, since tor_free() sets its
input to NULL, and has no effect when called with input NULL.)