Fix "config_error_nonbool" used with value instead of key

The function "config_error_nonbool", that is defined in "config.c",
is used to report an error when a config key in the config file
should have a corresponding value but it hasn't.

So the parameter to this function should be the key and not the
value, because the value is undefined. And it could crash if the
value is used.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-tag.c

@@ -260,7 +260,7 @@ static int git_tag_config(const char *var, const char *value, void *cb)
 {
 	if (!strcmp(var, "user.signingkey")) {
 		if (!value)
-			return config_error_nonbool(value);
+			return config_error_nonbool(var);
 		set_signingkey(value);
 		return 0;
 	}