H.264: Don't set data_partitioning for invalid NAL_DPA.

Before, the decoder could interpret a corrupt frame
as a NAL_DPA and NAL_DPC, and then start decoding
even if decode_slice_header() returned an error.
This frequently caused crashes.

Fixes issue1228, issue1229, and partially issue1238.

Originally committed as revision 19328 to svn://svn.ffmpeg.org/ffmpeg/trunk
