itu H.263: Fix handling of PB blocks

This fixes artifacts in Ticket744
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ituh263dec.c

@@ -566,11 +566,13 @@ static int h263_skip_b_part(MpegEncContext *s, int cbp)
 {
     LOCAL_ALIGNED_16(DCTELEM, dblock, [64]);
     int i, mbi;
+    int bli[6];
 
     /* we have to set s->mb_intra to zero to decode B-part of PB-frame correctly
      * but real value should be restored in order to be used later (in OBMC condition)
      */
     mbi = s->mb_intra;
+    memcpy(bli, s->block_last_index, sizeof(bli));
     s->mb_intra = 0;
     for (i = 0; i < 6; i++) {
         if (h263_decode_block(s, dblock, i, cbp&32) < 0)
@@ -578,6 +580,7 @@ static int h263_skip_b_part(MpegEncContext *s, int cbp)
         cbp+=cbp;
     }
     s->mb_intra = mbi;
+    memcpy(s->block_last_index, bli, sizeof(bli));
     return 0;
 }