replace if by assert as it should never by true

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@5080 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/avienc.c

@@ -517,8 +517,7 @@ static int avi_write_ix(AVFormatContext *s)
     unsigned char ix_tag[] = "ix00";
     int i, j;
 
-    if (url_is_streamed(pb))
-        return -1;
+    assert(!url_is_streamed(pb));
 
     if (avi->riff_id > AVI_MASTER_INDEX_SIZE)
         return -1;