patch 8.1.2091: double free when memory allocation fails

Problem:    Double free when memory allocation fails. (Zu-Ming Jiang)
Solution:   Use VIM_CLEAR() instead of vim_free(). (closes #4991)
diff --git a/src/version.c b/src/version.c
index e595c7a..2d9ffa3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2091,
+/**/
     2090,
 /**/
     2089,