iflib: Fix panic during driver reload stress test

During a driver reload stress test, after 50-300 reloads a panic occurs.
After adding sleeps in between loading and unloading the driver, the
issue does not occur. It's possible that loading/unloading too fast may
cause the gt_taskqueue pointer to be freed earlier than expected;
checking for a null pointer first fixes it.

Signed-off-by: Eric Joyner <erj@FreeBSD.org>

Reviewed by: erj@
Tested by: jeffrey.e.pieper@intel.com
Sponsored by: Intel Corporation
Differential Revision: https://reviews.freebsd.org/D39457

(cherry picked from commit 04d4e34538cf8f8ac99b5aa1c6c50b626d976dfd)