jpegparse: set framerate to 0/1 when duration is not known

Submitted by Nicola @drakkan

Link to original bug (#745724)

Description

Created attachment 298690
proposed fix

actually jpegparse set framerate to 1/1 when duration is not known, this seems wrong

Patch 298690, "proposed fix":
0001-jpegparse-set-framerate-0-1-when-duration-is-not-kno.patch