[settings] Avoid fetch_string_setting_copy() leaving an uninitialised pointer

For consistency with other functions in the fetch_setting() family,
ensure that fetch_string_setting_copy() always initialises the pointer
to the fetched setting even if fetching fails.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/core/settings.c

 	int len;
 	int check_len = 0;
 
+	*data = NULL;
+
 	len = fetch_setting_len ( settings, setting );
 	if ( len < 0 )
 		return len;