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/vim9cmds.c b/src/vim9cmds.c
index 90b61db..b9a0293 100644
--- a/src/vim9cmds.c
+++ b/src/vim9cmds.c
@@ -1420,7 +1420,7 @@
 	{
 	    semsg(_(e_separator_mismatch_str), p);
 	    vim_free(tofree);
-	    return FAIL;
+	    return NULL;
 	}
 	if (tofree == NULL)
 	    len = (int)(end - (p + 1));
@@ -1430,9 +1430,9 @@
 	vim_free(tofree);
 	p += len + 2 + dropped;
 	if (pat == NULL)
-	    return FAIL;
+	    return NULL;
 	if (generate_PUSHS(cctx, &pat) == FAIL)
-	    return FAIL;
+	    return NULL;
 
 	if (generate_COMPARE(cctx, EXPR_MATCH, FALSE) == FAIL)
 	    return NULL;