patch 8.1.1968: crash when using nested map()

Problem:    Crash when using nested map().
Solution:   Clear the pointer in prepare_vimvar(). (Ozaki Kiichi,
            closes #4890, closes #4891)
diff --git a/src/version.c b/src/version.c
index f5c9eec..5dbdc9f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1968,
+/**/
     1967,
 /**/
     1966,