auth: proper fixup for optional method parameter in pv_www_authenticate()

- reported by Muhammad Shahzad

modules/auth/auth_mod.c

@@ -830,6 +830,7 @@ static int fixup_pv_auth(void **param, int param_no)
 	switch(param_no) {
 		case 1:
 		case 2:
+		case 4:
 			return fixup_var_pve_str_12(param, 1);
 		case 3:
 			return fixup_var_int_12(param, 1);