Ensure consisten stream state before calling its ctl method.

Prior to calling ctl method, mu_stream_ioctl flushes stream
buffer.

* libmailutils/stream/fltstream.c (filter_ctl)
<MU_IOCTL_FILTER_SET_DISABLED>: Flush internal buffer prior to
setting the flag.
* libmailutils/stream/stream.c (mu_stream_ioctl): Flush stream
buffer.

@@ -399,6 +399,7 @@ filter_ctl (struct _mu_stream *stream, int code, int opcode, void *ptr)
 {
   struct _mu_filter_stream *fs = (struct _mu_filter_stream *)stream;
   int status;
+  size_t dummy;
   
   switch (code)
     {
@@ -406,6 +407,10 @@ filter_ctl (struct _mu_stream *stream, int code, int opcode, void *ptr)
       switch (opcode)
 	{
 	case MU_IOCTL_FILTER_SET_DISABLED:
+	  status = filter_write_internal (stream, mu_filter_flush,
+					  NULL, 0, &dummy);
+	  if (status)
+	    return status;
 	  if (ptr && *(int*)ptr)
 	    fs->fltflag |= _MU_FILTER_DISABLED;
 	  else

@@ -1156,7 +1156,10 @@ mu_stream_size (mu_stream_t stream, mu_off_t *psize)
 int
 mu_stream_ioctl (mu_stream_t stream, int family, int opcode, void *ptr)
 {
+  int rc;
   _bootstrap_event (stream);
+  if ((rc = _stream_flush_buffer (stream, _MU_STR_FLUSH_ALL)))
+    return rc;
   if (stream->ctl == NULL)
     return ENOSYS;
   return stream->ctl (stream, family, opcode, ptr);