notes.c: fix a segfault in notes_display_config()

If notes.displayRef is configured with no value[1], control should be
returned to the caller when notes.c:notes_display_config() checks if 'v'
is NULL. Otherwise, both git log --notes and git diff-tree --notes will
subsequently segfault when refs.h:has_glob_specials() calls strpbrk()
with a NULL first argument.

[1] Examples:
.git/config:
[notes]
	displayRef
$ git -c notes.displayRef [...]

Signed-off-by: Nate Avers <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: nateavers

notes.c

@@ -967,7 +967,7 @@ static int notes_display_config(const char *k, const char *v, void *cb)
 
 	if (*load_refs && !strcmp(k, "notes.displayref")) {
 		if (!v)
-			config_error_nonbool(k);
+			return config_error_nonbool(k);
 		string_list_add_refs_by_glob(&display_notes_refs, v);
 	}