mpegvideo: set correct offset for edge emulation buffer.

Using the old code, half of it was unused and the other half was too
small for e.g. >8bpp interlaced data, causing random buffer overruns.

libavcodec/mpegvideo.c

@@ -377,8 +377,7 @@ static int init_duplicate_context(MpegEncContext *s, MpegEncContext *base){
     int i;
 
     // edge emu needs blocksize + filter length - 1 (=17x17 for halfpel / 21x21 for h264)
-    FF_ALLOCZ_OR_GOTO(s->avctx, s->allocated_edge_emu_buffer, (s->width+64)*2*21*2, fail); //(width + edge + align)*interlaced*MBsize*tolerance
-    s->edge_emu_buffer= s->allocated_edge_emu_buffer + (s->width+64)*2*21;
+    FF_ALLOCZ_OR_GOTO(s->avctx, s->edge_emu_buffer, (s->width+64)*2*21*2, fail); //(width + edge + align)*interlaced*MBsize*tolerance
 
      //FIXME should be linesize instead of s->width*2 but that is not known before get_buffer()
     FF_ALLOCZ_OR_GOTO(s->avctx, s->me.scratchpad,  (s->width+64)*4*16*2*sizeof(uint8_t), fail)
@@ -416,7 +415,7 @@ fail:
 static void free_duplicate_context(MpegEncContext *s){
     if(s==NULL) return;
 
-    av_freep(&s->allocated_edge_emu_buffer); s->edge_emu_buffer= NULL;
+    av_freep(&s->edge_emu_buffer);
     av_freep(&s->me.scratchpad);
     s->me.temp=
     s->rd_scratchpad=
@@ -433,7 +432,6 @@ static void free_duplicate_context(MpegEncContext *s){
 
 static void backup_duplicate_context(MpegEncContext *bak, MpegEncContext *src){
 #define COPY(a) bak->a= src->a
-    COPY(allocated_edge_emu_buffer);
     COPY(edge_emu_buffer);
     COPY(me.scratchpad);
     COPY(me.temp);

libavcodec/mpegvideo.h

@@ -319,8 +319,7 @@ typedef struct MpegEncContext {
     uint8_t *mbintra_table;       ///< used to avoid setting {ac, dc, cbp}-pred stuff to zero on inter MB decoding
     uint8_t *cbp_table;           ///< used to store cbp, ac_pred for partitioned decoding
     uint8_t *pred_dir_table;      ///< used to store pred_dir for partitioned decoding
-    uint8_t *allocated_edge_emu_buffer;
-    uint8_t *edge_emu_buffer;     ///< points into the middle of allocated_edge_emu_buffer
+    uint8_t *edge_emu_buffer;     ///< temporary buffer for if MVs point to out-of-frame data
     uint8_t *rd_scratchpad;       ///< scratchpad for rate distortion mb decision
     uint8_t *obmc_scratchpad;
     uint8_t *b_scratchpad;        ///< scratchpad used for writing into write only buffers