notes-utils: handle boolean notes.rewritemode correctly

If we carry on after outputting config_error_nonbool then we're
guaranteed to dereference a null pointer.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes-utils.c

diff --git a/notes-utils.c b/notes-utils.c
index 2975dcd..4aa7023 100644
--- a/notes-utils.c
+++ b/notes-utils.c
@@ -75,7 +75,7 @@
 		return 0;
 	} else if (!c->mode_from_env && !strcmp(k, "notes.rewritemode")) {
 		if (!v)
-			config_error_nonbool(k);
+			return config_error_nonbool(k);
 		c->combine = parse_combine_notes_fn(v);
 		if (!c->combine) {
 			error(_("Bad notes.rewriteMode value: '%s'"), v);