media: cx88: Fix a null-ptr-deref bug in buffer_prepare()

When the driver calls cx88_risc_buffer() to prepare the buffer, the
function call may fail, resulting in a empty buffer and null-ptr-deref
later in buffer_queue().

The following log can reveal it:

[   41.822762] general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN PTI
[   41.824488] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[   41.828027] RIP: 0010:buffer_queue+0xc2/0x500
[   41.836311] Call Trace:
[   41.836945]  __enqueue_in_driver+0x141/0x360
[   41.837262]  vb2_start_streaming+0x62/0x4a0
[   41.838216]  vb2_core_streamon+0x1da/0x2c0
[   41.838516]  __vb2_init_fileio+0x981/0xbc0
[   41.839141]  __vb2_perform_fileio+0xbf9/0x1120
[   41.840072]  vb2_fop_read+0x20e/0x400
[   41.840346]  v4l2_read+0x215/0x290
[   41.840603]  vfs_read+0x162/0x4c0

Fix this by checking the return value of cx88_risc_buffer()

[hverkuil: fix coding style issues]

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>