avdevice/v4l2: only use frame period from v4l2 if valid

There is evidence that some drivers do not set a valid value

See: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=718805
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avdevice/v4l2: only use average frame rate if set

It might be unset on some platforms with some drivers and some input

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>