net-snmp: fix CVE-2014-2285

The perl_trapd_handler function in perl/TrapReceiver/TrapReceiver.xs in
Net-SNMP 5.7.3.pre3 and earlier, when using certain Perl versions, allows
remote attackers to cause a denial of service (snmptrapd crash) via an
empty community string in an SNMP trap, which triggers a NULL pointer
dereference within the newSVpv function in Perl.

Refer to: https://bugzilla.redhat.com/show_bug.cgi?id=1072044



Signed-off-by: Junling Zheng <zhengjunling@huawei.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>