virtio_pci: fix a NULL pointer reference in vp_del_vqs

virtio_pci: fix a NULL pointer reference in vp_del_vqs

If the msix_affinity_masks is alloced failed, then we'll
try to free some resources in vp_free_vectors() that may
access it directly.

We met the following stack in our production:
[   29.296767] BUG: unable to handle kernel NULL pointer dereference at  (null)
[   29.311151] IP: [<ffffffffc04fe35a>] vp_free_vectors+0x6a/0x150 [virtio_pci]
[   29.324787] PGD 0
[   29.333224] Oops: 0000 [#1] SMP
[...]
[   29.425175] RIP: 0010:[<ffffffffc04fe35a>]  [<ffffffffc04fe35a>] vp_free_vectors+0x6a/0x150 [virtio_pci]
[   29.441405] RSP: 0018:ffff9a55c2dcfa10  EFLAGS: 00010206
[   29.453491] RAX: 0000000000000000 RBX: ffff9a55c322c400 RCX: 0000000000000000
[   29.467488] RDX: 0000000000000000 RSI: 0000000000000000 RDI: ffff9a55c322c400
[   29.481461] RBP: ffff9a55c2dcfa20 R08: 0000000000000000 R09: ffffc1b6806ff020
[   29.495427] R10: 0000000000000e95 R11: 0000000000aaaaaa R12: 0000000000000000
[   29.509414] R13: 0000000000010000 R14: ffff9a55bd2d9e98 R15: ffff9a55c322c400
[   29.523407] FS:  00007fdcba69f8c0(0000) GS:ffff9a55c2840000(0000) knlGS:0000000000000000
[   29.538472] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[   29.551621] CR2: 0000000000000000 CR3: 000000003ce52000 CR4: 00000000003607a0
[   29.565886] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   29.580055] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[   29.594122] Call Trace:
[   29.603446]  [<ffffffffc04fe8a2>] vp_request_msix_vectors+0xe2/0x260 [virtio_pci]
[   29.618017]  [<ffffffffc04fedc5>] vp_try_to_find_vqs+0x95/0x3b0 [virtio_pci]
[   29.632152]  [<ffffffffc04ff117>] vp_find_vqs+0x37/0xb0 [virtio_pci]
[   29.645582]  [<ffffffffc057bf63>] init_vq+0x153/0x260 [virtio_blk]
[   29.658831]  [<ffffffffc057c1e8>] virtblk_probe+0xe8/0x87f [virtio_blk]
[...]

Cc: Gonglei <arei.gonglei@huawei.com>
Signed-off-by: Longpeng <longpeng2@huawei.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>