End startcode prefix search at the end of a AVC unit.

Fixes issue1550.

Originally committed as revision 20784 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -7521,7 +7521,7 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
             next_avc= buf_index + nalsize;
         } else {
             // start code prefix search
-            for(; buf_index + 3 < buf_size; buf_index++){
+            for(; buf_index + 3 < next_avc; buf_index++){
                 // This should always succeed in the first iteration.
                 if(buf[buf_index] == 0 && buf[buf_index+1] == 0 && buf[buf_index+2] == 1)
                     break;
@@ -7530,6 +7530,7 @@ static int decode_nal_units(H264Context *h, const uint8_t *buf, int buf_size){
             if(buf_index+3 >= buf_size) break;
 
             buf_index+=3;
+            if(buf_index >= next_avc) continue;
         }
 
         hx = h->thread_context[context_count];