V4L/DVB: stv6110x: Fix kernel null pointer deref

V4L/DVB: stv6110x: Fix kernel null pointer deref

The following OOPS happened when plugging two TT s2-1600:

[   96.521023] saa7146: register extension 'budget dvb'.
[   96.521052] budget dvb 0000:05:00.0: PCI INT A -> GSI 16 (level, low) -> IRQ 16
[   96.521070] IRQ 16/: IRQF_DISABLED is not guaranteed on shared IRQs
[   96.521076] saa7146: found saa7146 @ mem ffffc90011182c00 (revision 1, irq 16) (0x13c2,0x101c).
[   96.521080] saa7146 (0): dma buffer size 192512
[   96.521081] DVB: registering new adapter (TT-Budget S2-1600 PCI)
[   96.539929] adapter has MAC addr = 00:d0:5c:cc:b0:a2
[   96.890149] stv6110x_attach: Attaching STV6110x
[   96.912516] DVB: registering adapter 0 frontend 0 (STV090x Multistandard)...
[   96.912600] budget dvb 0000:05:01.0: PCI INT A -> GSI 17 (level, low) -> IRQ 17
[   96.912639] IRQ 17/: IRQF_DISABLED is not guaranteed on shared IRQs
[   96.912667] saa7146: found saa7146 @ mem ffffc90011314800 (revision 1, irq 17) (0x13c2,0x101c).
[   96.912673] saa7146 (1): dma buffer size 192512
[   96.912676] DVB: registering new adapter (TT-Budget S2-1600 PCI)
[   96.930893] adapter has MAC addr = 00:d0:5c:cc:b0:a3
[   97.233478] BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
[   97.233647] IP: [<ffffffffa029c450>] stv6110x_set_mode+0x70/0x80 [stv6110x]
[   97.233753] PGD 3c16f067 PUD 3c383067 PMD 0
[   97.234147] CPU 0
[   97.234246] Pid: 5200, comm: modprobe Not tainted 2.6.33.2 #1 P5QSE/P5Q SE
[   97.234317] RIP: 0010:[<ffffffffa029c450>]  [<ffffffffa029c450>] stv6110x_set_mode+0x70/0x80 [stv6110x]
[   97.234456] RSP: 0018:ffff88003c125c98  EFLAGS: 00010246
[   97.234461] RAX: ffffffffa029c460 RBX: ffff88003f84d800 RCX: ffff88003a19e140
[   97.234461] RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000
[   97.234461] RBP: ffff88003f84d828 R08: 0000000000000002 R09: 0000000000000004
[   97.234461] R10: 0000000000000003 R11: 0000000000000010 R12: ffff88003f84d800
[   97.234461] R13: ffff88003f84d828 R14: ffff88003f84d828 R15: 0000000000000001
[   97.234461] FS:  00007f9f7253e6f0(0000) GS:ffff880001800000(0000) knlGS:0000000000000000
[   97.234461] CS:  0010 DS: 0000 ES: 0000 CR0: 000000008005003b
[   97.234461] CR2: 0000000000000010 CR3: 000000003c382000 CR4: 00000000000006b0
[   97.234461] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[   97.234461] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[   97.234461] Process modprobe (pid: 5200, threadinfo ffff88003c124000, task ffff88003e893ac0)
[   97.234461]  ffff88003f84d800 ffff88003f84d828 ffff88003f84d800 ffffffffa0292343
[   97.234461] <0> ffff88003f84d828 ffff88003ef70ae0 ffffffffa0280800 ffffffffa02934d2
[   97.234461] <0> ffffffffa0295260 0000000000000000 ffffffffa02948b0 ffff88003df79800
[   97.234461]  [<ffffffffa0292343>] ? stv090x_sleep+0x33/0x120 [stv090x]
[   97.234461]  [<ffffffffa02934d2>] ? stv090x_attach+0x1e2/0x73c [stv090x]
[   97.234461]  [<ffffffff81007cc5>] ? dma_generic_alloc_coherent+0xa5/0x160
[   97.234461]  [<ffffffffa026e1f5>] ? saa7146_init_one+0x7d5/0x910 [saa7146]
[   97.234461]  [<ffffffff811b84b2>] ? local_pci_probe+0x12/0x20
[   97.234461]  [<ffffffff811b87d0>] ? pci_device_probe+0x110/0x120
[   97.234461]  [<ffffffff81221788>] ? driver_probe_device+0x98/0x1b0
[   97.234461]  [<ffffffff81221933>] ? __driver_attach+0x93/0xa0
[   97.234461]  [<ffffffff812218a0>] ? __driver_attach+0x0/0xa0
[   97.234461]  [<ffffffff81220f18>] ? bus_for_each_dev+0x58/0x80
[   97.234461]  [<ffffffff8122079d>] ? bus_add_driver+0x14d/0x280
[   97.234461]  [<ffffffffa0284000>] ? budget_init+0x0/0xc [budget]
[   97.234461]  [<ffffffff81221c29>] ? driver_register+0x79/0x170
[   97.234461]  [<ffffffffa0284000>] ? budget_init+0x0/0xc [budget]
[   97.234461]  [<ffffffff811b8a48>] ? __pci_register_driver+0x58/0xe0
[   97.234461]  [<ffffffffa0284000>] ? budget_init+0x0/0xc [budget]
[   97.234461]  [<ffffffff810001d5>] ? do_one_initcall+0x35/0x190
[   97.234461]  [<ffffffff81063d37>] ? sys_init_module+0xe7/0x260
[   97.234461]  [<ffffffff8100256b>] ? system_call_fastpath+0x16/0x1b
[   97.234461] RIP  [<ffffffffa029c450>] stv6110x_set_mode+0x70/0x80 [stv6110x]
[   97.234461]  RSP <ffff88003c125c98>
[   97.240074] ---[ end trace b53ecbbbbef15e99 ]---

Prevents calling stv6110x_set_mode() if fe->tuner_priv is not defined,
in order to avoid the above bug.

Signed-off-by: Guy Martin <gmsoft@tuxicoman.be>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>