Fix bug where the avi demxuer lost the first frame of malformed

ODML avis.
Fixes VS2k5DebugDemo-01-partial.avi

Originally committed as revision 16167 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avidec.c

@@ -163,7 +163,7 @@ static int read_braindead_odml_indx(AVFormatContext *s, int frame_num){
 #endif
             if(last_pos == pos || pos == base - 8)
                 avi->non_interleaved= 1;
-            else
+            if(last_pos != pos)
                 av_add_index_entry(st, pos, ast->cum_len / FFMAX(1, ast->sample_size), len, 0, key ? AVINDEX_KEYFRAME : 0);
 
             if(ast->sample_size)