scsi: mpt3sas: Fix double free warnings

Fix following warning from Smatch static analyser:

drivers/scsi/mpt3sas/mpt3sas_base.c:5256 _base_allocate_memory_pools()
warn: 'ioc->hpr_lookup' double freed

drivers/scsi/mpt3sas/mpt3sas_base.c:5256 _base_allocate_memory_pools()
warn: 'ioc->internal_lookup' double freed

Link: https://lore.kernel.org/r/20200508110738.30732-1-suganath-prabu.subramani@broadcom.comReported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Suganath Prabu S <suganath-prabu.subramani@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

drivers/scsi/mpt3sas/mpt3sas_base.c

@@ -4899,7 +4899,9 @@ _base_release_memory_pools(struct MPT3SAS_ADAPTER *ioc)
 	}
 
 	kfree(ioc->hpr_lookup);
+	ioc->hpr_lookup = NULL;
 	kfree(ioc->internal_lookup);
+	ioc->internal_lookup = NULL;
 	if (ioc->chain_lookup) {
 		for (i = 0; i < ioc->scsiio_depth; i++) {
 			for (j = ioc->chains_per_prp_buffer;