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)

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1968,
 /**/
     1967,
 /**/