In video4linux2, in the case the timebase value in ap is 0/0, read the
timebase value already set in the driver, and set it back in the codec
stream, rather than leaving the invalid value of 0/0.

In particular, fix ffmpeg grabbing timestamps when the timebase value
is not set through the CLI.

Originally committed as revision 26224 to svn://svn.ffmpeg.org/ffmpeg/trunk
