libavdevice/v4l2: fix descriptors leak on error paths

Signed-off-by: Dmitry Volytnsev <xeioexception@gmail.com>
Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Reviewed-by: Giorgio Vazzana <mywing81@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>