ftmod_misdn: Reserve some space for the mISDN message header in misdn_write().

Signed-off-by: Stefan Knoblich <stkn@openisdn.net>

libs/freetdm/src/ftmod/ftmod_misdn/ftmod_misdn.c

@@ -1280,7 +1280,7 @@ static FIO_WRITE_FUNCTION(misdn_write)
 	hh->id   = MISDN_ID_ANY;
 
 	/* avoid buffer overflow */
-	size = MIN(size, MAX_DATA_MEM);
+	size = MIN(size, MAX_DATA_MEM - MISDN_HEADER_LEN);
 
 	memcpy(wbuf + MISDN_HEADER_LEN, data, size);
 	size += MISDN_HEADER_LEN;