ibmvnic: fix NULL pointer dereference in ibmvic_reset_crq

crq->msgs could be NULL if the previous reset did not complete after
freeing crq->msgs. Check for NULL before dereferencing them.

Snippet of call trace:
...
ibmvnic 30000003 env3 (unregistering): Releasing sub-CRQ
ibmvnic 30000003 env3 (unregistering): Releasing CRQ
BUG: Kernel NULL pointer dereference on read at 0x00000000
Faulting instruction address: 0xc0000000000c1a30
Oops: Kernel access of bad area, sig: 11 [#1]
LE PAGE_SIZE=64K MMU=Hash SMP NR_CPUS=2048 NUMA pSeries
Modules linked in: ibmvnic(E-) rpadlpar_io rpaphp xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT nf_reject_ipv4 nft_compat nft_counter nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nf_tables xsk_diag tcp_diag udp_diag tun raw_diag inet_diag unix_diag bridge af_packet_diag netlink_diag stp llc rfkill sunrpc pseries_rng xts vmx_crypto uio_pdrv_genirq uio binfmt_misc ip_tables xfs libcrc32c sd_mod t10_pi sg ibmvscsi ibmveth scsi_transport_srp dm_mirror dm_region_hash dm_log dm_mod [last unloaded: ibmvnic]
CPU: 20 PID: 8426 Comm: kworker/20:0 Tainted: G            E     5.10.0-rc1+ #12
Workqueue: events __ibmvnic_reset [ibmvnic]
NIP:  c0000000000c1a30 LR: c008000001b00c18 CTR: 0000000000000400
REGS: c00000000d05b7a0 TRAP: 0380   Tainted: G            E      (5.10.0-rc1+)
MSR:  800000000280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 44002480  XER: 20040000
CFAR: c0000000000c19ec IRQMASK: 0
GPR00: 0000000000000400 c00000000d05ba30 c008000001b17c00 0000000000000000
GPR04: 0000000000000000 0000000000000000 0000000000000000 00000000000001e2
GPR08: 000000000001f400 ffffffffffffd950 0000000000000000 c008000001b0b280
GPR12: c0000000000c19c8 c00000001ec72e00 c00000000019a778 c00000002647b440
GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
GPR20: 0000000000000006 0000000000000001 0000000000000003 0000000000000002
GPR24: 0000000000001000 c008000001b0d570 0000000000000005 c00000007ab5d550
GPR28: c00000007ab5c000 c000000032fcf848 c00000007ab5cc00 c000000032fcf800
NIP [c0000000000c1a30] memset+0x68/0x104
LR [c008000001b00c18] ibmvnic_reset_crq+0x70/0x110 [ibmvnic]
Call Trace:
[c00000000d05ba30] [0000000000000800] 0x800 (unreliable)
[c00000000d05bab0] [c008000001b0a930] do_reset.isra.40+0x224/0x634 [ibmvnic]
[c00000000d05bb80] [c008000001b08574] __ibmvnic_reset+0x17c/0x3c0 [ibmvnic]
[c00000000d05bc50] [c00000000018d9ac] process_one_work+0x2cc/0x800
[c00000000d05bd20] [c00000000018df58] worker_thread+0x78/0x520
[c00000000d05bdb0] [c00000000019a934] kthread+0x1c4/0x1d0
[c00000000d05be20] [c00000000000d5d0] ret_from_kernel_thread+0x5c/0x6c

Fixes: 032c5e82 ("Driver for IBM System i/p VNIC protocol")
Signed-off-by: Lijun Pan <ljp@linux.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>