net-snmp: fix snmpd crash via AgentX connection

backport a patch to fixed snmpd crashing when an AgentX
subagent disconnect in the middle of processing of a request.

Signed-off-by: Zhu Yanjun <yanjun.zhu@windriver.com>
Signed-off-by: Jackie Huang <jackie.huang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>
(cherry picked from commit 582eddb44e0d1ce67a04afb441059a8bf7cc1ef1)
Signed-off-by: Armin Kuster <akuster808@gmail.com>