vallen is verified to be less than len, therefore, it can never

be the case that `vallen >= len + sizeof(rhostname)`.

This PR fixes the check so the `rhostname` array does not overflow.

Reported-by: Github Security Lab <securitylab@github.com>
Signed-off-by: Alvaro Muñoz <pwntester@github.com>

Author: ghsecuritylab

external/mbed-os/features/FEATURE_LWIP/lwip-interface/lwip/src/netif/ppp/lwip_eap.c

@@ -1417,7 +1417,7 @@ static void eap_request(ppp_pcb *pcb, u_char *inp, int id, int len) {
 		}
 
 		/* Not so likely to happen. */
-		if (vallen >= len + sizeof (rhostname)) {
+		if (len - vallen >= sizeof (rhostname)) {
 			ppp_dbglog("EAP: trimming really long peer name down");
 			MEMCPY(rhostname, inp + vallen, sizeof (rhostname) - 1);
 			rhostname[sizeof (rhostname) - 1] = '\0';
@@ -1845,7 +1845,7 @@ static void eap_response(ppp_pcb *pcb, u_char *inp, int id, int len) {
 		}
 
 		/* Not so likely to happen. */
-		if (vallen >= len + sizeof (rhostname)) {
+		if (len - vallen >= sizeof (rhostname)) {
 			ppp_dbglog("EAP: trimming really long peer name down");
 			MEMCPY(rhostname, inp + vallen, sizeof (rhostname) - 1);
 			rhostname[sizeof (rhostname) - 1] = '\0';