ffm: disable adjust_write_index()

This code can in its current form not work with ffserver
Fixes Ticket1249
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/ffmdec.c

@@ -285,7 +285,7 @@ static int ffm_read_header(AVFormatContext *s)
     /* get also filesize */
     if (pb->seekable) {
         ffm->file_size = avio_size(pb);
-        if (ffm->write_index)
+        if (ffm->write_index && 0)
             adjust_write_index(s);
     } else {
         ffm->file_size = (UINT64_C(1) << 63) - 1;