ibmvnic: avoid memset null scrq msgs

scrq->msgs could be NULL during device reset, causing Linux to crash.
So, check before memset scrq->msgs.

Fixes: c8b2ad0a

 ("ibmvnic: Sanitize entire SCRQ buffer on reset")
Signed-off-by: Dany Madden <drt@linux.ibm.com>
Signed-off-by: Lijun Pan <ljp@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>