net-snmp: don't return incompletely parsed varbinds

the snmp_pdu_parse() function could leave incompletely parsed varBind
variables in the list of variables in case the parsing of the SNMP
PDU failed. If later processing tries to operate on the stale and
incompletely processed varBind (e.g. when printing the variables),
this can lead to e.g. crashes or, possibly, execution of arbitrary
code.

The snmp_pdu_parse() function stores varBind variables in a list of
netsnmp_variable_list structures. Each time the function parses a new
varBind, a new netsnmp_variable_list item is allocated on the heap
and linked to the list of variables. The problem is that this item
is not removed from the list, even if snmp_pdu_parse() fails to
complete the parsing.

The "type" member of the stale netsnmp_variable_list is not
properly initialized in case snmp_pdu_parse() returns early from the
parsing. However, the "type" member is used to determine later code
paths, which is why we see crashes in a variety of functions,
although the root cause for all of these is the same.

This patch come from
http://sourceforge.net/p/net-snmp/code/ci/f23bcd3ac6ddee5d0a48f9703007ccc738914791/



Written-by: Robert Story
Signed-off-by: Jian Liu <jian.liu@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Joe MacDonald <joe_macdonald@mentor.com>