Fix uninitialized mutex usage in synctask_destroy

synctask_new() initialize task->mutex is task->synccbk is NULL.
synctask_done() calls synctask_destroy() if task->synccbk is not NULL.
synctask_destroy() always destroys the mutex.

Fix that by checking for task->synccbk in synctask_destroy()

This is a backport of I50bb53bc6e2738dc0aa830adc4c1ea37b24ee2a0

BUG: 764655
Change-Id: I3d6292f05a986ae3ceee35161791348ce3771c12
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/4920
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>