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)

src/version.c

@@ -707,6 +707,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    339,
 /**/
     338,
 /**/