ext/jpeg/gstjpegdec.*: Fix crashes/invalid memory access for pictures that have a...

ext/jpeg/gstjpegdec.*: Fix crashes/invalid memory access for pictures that have a height that is not a multiple of 16...

Original commit message from CVS:
* ext/jpeg/gstjpegdec.c: (gst_jpeg_dec_init), (gst_jpeg_dec_chain),
(gst_jpeg_dec_change_state):
* ext/jpeg/gstjpegdec.h:
Fix crashes/invalid memory access for pictures that have a height
that is not a multiple of 16 (or rather: v_samp_factor * DCTSIZE).
Also fix the state change function for downwards state changes
(need to chain up to parent before destroying our resources, to
make sure pads get deactivated and our chain function isn't
running and using those very same resources in another thread).
The jpeg line buffer only needs to be v_samp_factor*DCTSIZE lines
per plane, not picture_height lines; allocate that on the stack.