patch 9.0.0864: crash when using "!!" without a previous shell command

Problem:    Crash when using "!!" without a previous shell command.
Solution:   Check "prevcmd" is not NULL. (closes #11487)
diff --git a/src/version.c b/src/version.c
index 64ed648..a0c4371 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    864,
+/**/
     863,
 /**/
     862,