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.)

src/or/command.c

@@ -340,7 +340,6 @@ command_process_create_cell(cell_t *cell, channel_t *chan)
     if (len < 0) {
       log_warn(LD_OR,"Failed to generate key material. Closing.");
       circuit_mark_for_close(TO_CIRCUIT(circ), END_CIRC_REASON_INTERNAL);
-      tor_free(create_cell);
       return;
     }
     created_cell.cell_type = CELL_CREATED_FAST;