Check the return value of device_try_init() immediately after calling such a
function. Avoids the following warning:
libavdevice/v4l2.c: In function ‘v4l2_read_header’:
libavdevice/v4l2.c:586: warning: ‘codec_id’ may be used uninitialized in this function

Originally committed as revision 22986 to svn://svn.ffmpeg.org/ffmpeg/trunk
