Description of problem:

On debug build, any gluster volume set with an invalid option crashes with the following backtrace:

#0  0x00007f1a8ca280b0 in pthread_spin_lock () from /lib64/libpthread.so.0
#1  0x00007f1a8d900b44 in __gf_free (free_ptr=0x7f1a680293b0) at mem-pool.c:303
#2  0x00007f1a82f508da in gd_sync_task_begin (op_ctx=0x7f1a68000bac, req=0x7f1a740019ac) at glusterd-syncop.c:1767
#3  0x00007f1a82f50970 in glusterd_op_begin_synctask (req=0x7f1a740019ac, op=GD_OP_SET_VOLUME, dict=0x7f1a68000bac)
    at glusterd-syncop.c:1787
#4  0x00007f1a82eb39a4 in __glusterd_handle_set_volume (req=0x7f1a740019ac) at glusterd-handler.c:1871
#5  0x00007f1a82eae740 in glusterd_big_locked_handler (req=0x7f1a740019ac, actor_fn=0x7f1a82eb3488 <__glusterd_handle_set_volume>)
    at glusterd-handler.c:83
#6  0x00007f1a82eb3a8f in glusterd_handle_set_volume (req=0x7f1a740019ac) at glusterd-handler.c:1893
#7  0x00007f1a8d913069 in synctask_wrap (old_task=0x7f1a74002d60) at syncop.c:375
#8  0x00007f1a8c2b9000 in ?? () from /lib64/libc.so.6
#9  0x0000000000000000 in ?? ()


Version-Release number of selected component (if applicable):
Mainline

How reproducible:
Always

Steps to Reproduce:
1. Create & start any type of volume
2. gluster volume set <VOLNAME> barrier abcd


Actual results:
Glusterd crashed

Expected results:
Glusterd should not crash

Additional info: