brcmfmac: avoid null pointer access when brcmf_msgbuf_get_pktid() fails

The function brcmf_msgbuf_get_pktid() may return a NULL pointer so
the callers should check the return pointer before accessing it to
avoid the crash below (see [1]):

brcmfmac: brcmf_msgbuf_get_pktid: Invalid packet id 273 (not in use)
BUG: unable to handle kernel NULL pointer dereference at 0000000000000080
IP: [<ffffffff8145b225>] skb_pull+0x5/0x50
PGD 0
Oops: 0000 [#1] PREEMPT SMP
Modules linked in: pci_stub vboxpci(O) vboxnetflt(O) vboxnetadp(O) vboxdrv(O)
 snd_hda_codec_hdmi bnep mousedev hid_generic ushwmon msr ext4 crc16 mbcache
 jbd2 sd_mod uas usb_storage ahci libahci libata scsi_mod xhci_pci xhci_hcd
 usbcore usb_common
CPU: 0 PID: 1661 Comm: irq/61-brcmf_pc Tainted: G O    4.0.1-MacbookPro-ARCH #1
Hardware name: Apple Inc. MacBookPro12,1/Mac-E43C1C25D4880AD6,
 BIOS MBP121.88Z.0167.B02.1503241251 03/24/2015
task: ffff880264203cc0 ti: ffff88025ffe4000 task.ti: ffff88025ffe4000
RIP: 0010:[<ffffffff8145b225>]  [<ffffffff8145b225>] skb_pull+0x5/0x50
RSP: 0018:ffff88025ffe7d40  EFLAGS: 00010202
RAX: 0000000000000000 RBX: ffff88008a33c000 RCX: 0000000000000044
RDX: 0000000000000000 RSI: 000000000000004a RDI: 0000000000000000
RBP: ffff88025ffe7da8 R08: 0000000000000096 R09: 000000000000004a
R10: 0000000000000000 R11: 000000000000048e R12: ffff88025ff14f00
R13: 0000000000000000 R14: ffff880263b48200 R15: ffff88008a33c000
FS:  0000000000000000(0000) GS:ffff88026ec00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000080 CR3: 000000000180b000 CR4: 00000000003407f0
Stack:
 ffffffffa06aed74 ffff88025ffe7dc8 ffff880263b48270 ffff880263b48278
 05ea88020000004a 0002ffff81014635 000000001720b2f6 ffff88026ec116c0
 ffff880263b48200 0000000000010000 ffff880263b4ae00 ffff880264203cc0
Call Trace:
 [<ffffffffa06aed74>] ? brcmf_msgbuf_process_rx+0x404/0x480 [brcmfmac]
 [<ffffffff810cea60>] ? irq_finalize_oneshot.part.30+0xf0/0xf0
 [<ffffffffa06afb55>] brcmf_proto_msgbuf_rx_trigger+0x35/0xf0 [brcmfmac]
 [<ffffffffa06baf2a>] brcmf_pcie_isr_thread_v2+0x8a/0x130 [brcmfmac]
 [<ffffffff810cea80>] irq_thread_fn+0x20/0x50
 [<ffffffff810ceddf>] irq_thread+0x13f/0x170
 [<ffffffff810cebf0>] ? wake_threads_waitq+0x30/0x30
 [<ffffffff810ceca0>] ? irq_thread_dtor+0xb0/0xb0
 [<ffffffff81092a08>] kthread+0xd8/0xf0
 [<ffffffff81092930>] ? kthread_create_on_node+0x1c0/0x1c0
 [<ffffffff8156d898>] ret_from_fork+0x58/0x90
 [<ffffffff81092930>] ? kthread_create_on_node+0x1c0/0x1c0
Code: 01 83 e2 f7 88 50 01 48 83 c4 08 5b 5d f3 c3 0f 1f 80 00 00 00 00 83 e2
 f7 88 50 01 c3 66 0f 1f 84 00 00 00 00 00 0f 1f
RIP  [<ffffffff8145b225>] skb_pull+0x5/0x50
 RSP <ffff88025ffe7d40>
CR2: 0000000000000080
---[ end trace b074c0f90e7c997d ]---

[1] http://mid.gmane.org/20150430193259.GA5630@googlemail.com

Cc: <stable@vger.kernel.org> # v3.18, v3.19, v4.0, v4.1
Reported-by: Michael Hornung <mhornung.linux@gmail.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>