builtin-tag.c: guard config parser from value=NULL

user.signingkey configuration expects a string value.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-tag.c

@@ -258,7 +258,7 @@ static int git_tag_config(const char *var, const char *value)
 {
 	if (!strcmp(var, "user.signingkey")) {
 		if (!value)
-			die("user.signingkey without value");
+			return config_error_nonbool(value);
 		set_signingkey(value);
 		return 0;
 	}