v4l2videodec: only set latency if the frame duration is valid

v4l2videodec: only set latency if the frame duration is valid

If the duration of the v4l2object is GST_CLOCK_TIME_NONE, because the
sink did not specify a framerate in the caps and the driver accepts the
framerate, the decoder element uses GST_CLOCK_TIME_NONE to calculate and
set the element latency.

While this is a bug of the capture driver, the decoder element should
not use the invalid duration to calculate a latency, but print a warning
instead.

https://bugzilla.gnome.org/show_bug.cgi?id=779466