Refactor: Fix problems found by clang.

• Several strings are initialized to some value, immediately followed by a block of code that sets them to some other value. The original initialization can go away.

• Warn if vsnprintf fails.

• Get rid of setting a couple return values that are never checked.

• In read_client, set msg to 0 if we malloc'd it so that header->length is an initialized value when we access it later on.

• In _find_myself, check that tb[IFA_ADDRESS] is not NULL before accessing it. I'm not sure what we can do if both this and tb[IFA_LOCAL] are NULL aside from log and return.