avoid overflow in snmp message id

[Rob Pierce]

Prevent server side (snmpd) overflow for message id in the snmp header.

ok?

Index: snmpclient.c
===================================================================
RCS file: /cvs/src/usr.sbin/snmpctl/snmpclient.c,v
retrieving revision 1.16
diff -u -p -r1.16 snmpclient.c
--- snmpclient.c        8 Aug 2018 18:50:38 -0000       1.16
+++ snmpclient.c        10 Aug 2018 14:21:23 -0000
@@ -407,7 +407,7 @@ snmpc_sendreq(struct snmpc *sc, unsigned
                erroridx = SNMPC_MAXREPETITIONS;
 
        /* SNMP header */
-       sc->sc_msgid = arc4random();
+       sc->sc_msgid = arc4random() & 0x7fffffff;
        if ((root = ber_add_sequence(NULL)) == NULL)
                return (-1);
        if ((b = ber_printf_elements(root, "ds{tddd{{O0}}",