package/netsnmp: fix memory leak in IP-MIB when running without IPv6

In a Linux system without IPv6 support (or booted with "ipv6.disable=1")
file /proc/net/snmp6 is not present. If such file is not present an allocated
memory is not freed. Memory leak occurs even without snmp queries.

Problem seen at least since netsnmp 5.7.3 (probably even v5.6.1).
Patch backported from netsnmp 5.9, where the problem does not appear any more.

Signed-off-by: Adam Wujek <dev_public@wujek.eu>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>