patch 8.2.4318: various comment and indent mistakes, returning wrong zero

Problem:    Various comment and indent mistakes, returning wrong zero.
Solution:   Fix the mistakes.  Return NULL instead of FAIL.
diff --git a/src/window.c b/src/window.c
index f80334d..4fb92e0 100644
--- a/src/window.c
+++ b/src/window.c
@@ -766,7 +766,7 @@
 }
 
 /*
- * If "split_disallowed" is set given an error and return FAIL.
+ * If "split_disallowed" is set give an error and return FAIL.
  * Otherwise return OK.
  */
     static int