snmptrapd: Fix an integer overflow

Do not add one to the trap type if doing this would cause an integer overflow.

Fixes: https://issues.oss-fuzz.com/issues/459029273

Author: bvanassche

apps/snmptrapd_handlers.c

@@ -1128,6 +1128,9 @@ snmp_input(int op, netsnmp_session *session,
             } else {
                 memcpy(trapOid, stdTrapOidRoot, sizeof(stdTrapOidRoot));
                 trapOidLen = OID_LENGTH(stdTrapOidRoot);  /* 9 */
+                /* Drop packets with an invalid trap type. */
+                if (pdu->trap_type == LONG_MAX)
+                    return 1;
                 trapOid[trapOidLen++] = pdu->trap_type+1;
             }
             break;