patch 9.0.0339: no check if the return value of XChangeGC() is NULL

Problem:    No check if the return value of XChangeGC() is NULL.
Solution:   Only use the return value when it is not NULL. (closes #11020)
diff --git a/src/version.c b/src/version.c
index 8091488..354a2e4 100644
--- a/src/version.c
+++ b/src/version.c
@@ -708,6 +708,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    339,
+/**/
     338,
 /**/
     337,