patch 8.0.1251: invalid expressin passed to WaitFor()

Problem:    Invalid expressin passed to WaitFor().
Solution:   Check if the variable exists.
diff --git a/src/version.c b/src/version.c
index e14c2fe..552de77 100644
--- a/src/version.c
+++ b/src/version.c
@@ -762,6 +762,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1251,
+/**/
     1250,
 /**/
     1249,