patch 8.1.1964: crash when using nested map() and filter()

Problem:    Crash when using nested map() and filter().
Solution:   Do not set the v:key type to string without clearing the pointer.

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1964,
 /**/
     1963,
 /**/