DPP: Fix error return value in dpp_auth_conf_rx()

Commit 03abb6b5416d472d473c7017802236f8397d0278 ('DPP: Reject unexpected
Req/Resp message based on Auth/PKEX role') used incorrect type of error
value (NULL vs. -1). Fix that.

Signed-off-by: Jouni Malinen <jouni@qca.qualcomm.com>

src/common/dpp.c

@@ -3665,7 +3665,7 @@ int dpp_auth_conf_rx(struct dpp_authentication *auth, const u8 *hdr,
 
 	if (auth->initiator) {
 		dpp_auth_fail(auth, "Unexpected Authentication Confirm");
-		return NULL;
+		return -1;
 	}
 
 	auth->waiting_auth_conf = 0;