Fix crash when system has no ipv6 address [BZ #17125]

Fix crash when system has no ipv6 address [BZ #17125]

Here's an updated patch to fix the crash in bug-ga2 when the system
has no configured ipv6 address.  I have taken a different approach of
using libc_freeres_fn instead of the libc_freeres_ptr since the former
gives better control over what is freed; we need that since cache may
or may not be allocated using malloc.

Verified that bug-ga2 works correctly in both cases and does not have
memory leaks in either of them.