jpegparse: Don't generate timestamp for 0/1 framerates

This fixes the issue reported on the mailing list by Guillermo Rodriguez Garcia:

I have been using gstreamer to stream live video from mjpeg cameras with a pipeline like this:

gst-launch-1.0 souphttpsrc is-live=true location=<camera_url> ! multipartdemux ! image/jpeg,framerate=0/1 ! jpegparse ! avdec_mjpeg ! videoconvert ! ximagesink

This was working fine with 1.14.x. Recently I upgraded gstreamer to a more recent version and the above does not work well anymore. Video stutters or freezes.

I ran git bisect and found commit 5c52f866 to be the culprit.

If I revert that commit, the pipeline works fine again.

Is this the expected behaviour? Can someone shed some light?