v4l2src: fix 'hang' with some cameras caused by bad timestamping if no framerate...

v4l2src: fix 'hang' with some cameras caused by bad timestamping if no framerate is available

For cameras/drivers that don't support e.g. VIDIOC_G_PARM we'd end up without
a framerate and would try to divide by 0, causing run-time warnings and all
frames to be timestamped with 0, which makes sinks that sync against the clock
drop them, causing 'hangs' (observed with the pwc driver and a Logitech QuickCam
Pro 4000). So if we do not know the framerate, simply don't adjust the
timestamps. Fixes #591451.