ext/jpeg/gstjpegdec.c: Fix invalid memory access for some odd-sized images (see image contained in quicktime stream i...

Original commit message from CVS:
* ext/jpeg/gstjpegdec.c: (gst_jpeg_dec_decode_direct),
(gst_jpeg_dec_chain):
Fix invalid memory access for some odd-sized images
(see image contained in quicktime stream in #327083);
use g_malloc() instead of g_alloca().