Patch for https://groups.google.com/d/msg/vim_dev/pUhcNbcb2eQ/VrKSeT-DeB8J

diff_and_E749_fix.patch

diff --git a/src/diff.c b/src/diff.c
--- a/src/diff.c
+++ b/src/diff.c
@@ -804,7 +804,7 @@
     for (idx_new = idx_orig + 1; idx_new < DB_COUNT; ++idx_new)
     {
 	buf = curtab->tp_diffbuf[idx_new];
-	if (buf == NULL)
+	if (buf == NULL || buf->b_ml.ml_mfp == NULL)
 	    continue;
 	if (diff_write(buf, tmp_new) == FAIL)
 	    continue;