rtph264depay: only guess AU boundaries when aren't indicated by marker

The marker bit isn't mandatory and we had in place code to guess AU
boundaries by detecting a new picture start. This guessing code
didn't work with interlaced content that has proper marker bits
to indicate the AU boundaries. It was leaking the first field buffer
and producing a corrupted output.

fixes: https://bugzilla.gnome.org/show_bug.cgi?id=728041