patch 8.1.0400: using freed memory with :diffget

Problem:    Using freed memory with :diffget.
Solution:   Skip ex_diffupdate() while updating diffs. (closes #3442)
diff --git a/src/version.c b/src/version.c
index f991cd0..d3cc195 100644
--- a/src/version.c
+++ b/src/version.c
@@ -795,6 +795,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    400,
+/**/
     399,
 /**/
     398,