media: cx23885: Fix a null-ptr-deref bug in buffer_prepare() and buffer_finish()

In the Linux kernel, the following vulnerability has been resolved:

media: cx23885: Fix a null-ptr-deref bug in buffer_prepare() and buffer_finish()

When the driver calls cx23885_risc_buffer() to prepare the buffer, the
function call dma_alloc_coherent may fail, resulting in a empty buffer
risc->cpu. Later when we free the buffer or access the buffer, null ptr
deref is triggered.

This bug is similar to the following one:
https://git.linuxtv.org/media_stage.git/commit/?id=2b064d91440b33fba5b452f2d1b31f13ae911d71.

We believe the bug can be also dynamically triggered from user side.
Similarly, we fix this by checking the return value of cx23885_risc_buffer()
and the value of risc->cpu before buffer free.