avcodec/cuvid: Always check for internal errors during parsing

The cuvid parser is basically undocumented, and although you'd
think that a failed callback would result in the overall parse
call returning an error, that is not true.

So, we end up silently trying to keep going as if nothing is wrong,
which doesn't achieve anything.

Solution: check the internal error flag every time.
Signed-off-by: Philip Langdale <philipl@overt.org>
Signed-off-by: Timo Rothenpieler <timo@rothenpieler.org>
