Reset state after some frame header is found.

Fixes issue704

Originally committed as revision 15742 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/aac_ac3_parser.c

@@ -49,6 +49,7 @@ get_next:
             if(len<=0){
                 i=END_NOT_FOUND;
             }else{
+                s->state=0;
                 i-= s->header_size -1;
                 s->remaining_size = len;
                 if(!new_frame_start || pc->index+i<=0){