ext/jpeg/gstjpegdec.*: Fix durations on outgoing buffers after seeking in MJPEG files (#334083); some minor clean-ups.

Original commit message from CVS:
* ext/jpeg/gstjpegdec.c: (gst_jpeg_dec_class_init),
(gst_jpeg_dec_chain), (gst_jpeg_dec_change_state):
* ext/jpeg/gstjpegdec.h:
Fix durations on outgoing buffers after seeking
in MJPEG files (#334083); some minor clean-ups.