v4l2: Add protection when set decoder capture fps accroding to output fps

v4l2: Add protection when set decoder capture fps accroding to output fps

Some v4l2 drivers don't have the capacity to change framerate. There is
chance to make decoder capture fps to be 0/0 if numerator and denominator
returned by G_PARM ioctl are both 0. It causes critical warning
"passed '0' as denominator for `GstFraction'".

In order to fix this, add protection when set decoder capture fps according
to output fps.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/1048>