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

libavcodec/h264.c

@@ -7603,9 +7603,12 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
             init_get_bits(&hx->s.gb, ptr, bit_length);
             hx->intra_gb_ptr=
             hx->inter_gb_ptr= NULL;
+
+            if ((err = decode_slice_header(hx, h)) < 0)
+                break;
+
             hx->s.data_partitioning = 1;
 
-            err = decode_slice_header(hx, h);
             break;
         case NAL_DPB:
             init_get_bits(&hx->intra_gb, ptr, bit_length);