The yoffset calculation in oc_dec_init_dummy_frame was incorrect.

This was only used when a stream started without a keyframe, and was
 initializing the wrong region of memory.
This was harmless (the region it initialized was still safely
 allocated, it just didn't cover all of the frame we wanted to use
 as a reference frame), but lead to unpredictable results and
 valgrind errors.
See https://bugzilla.mozilla.org/show_bug.cgi?id=625773

svn path=/trunk/theora/; revision=17780