network: don't return allocated buffer of zero length from deserialize_in_addrs()

deserialize_in_addrs() allocates the buffer before trying to parse
the IP address. Since a parsing error is silently ignored, the returned
size might be zero. In such a case we shouldn't return any buffer.

Anyway, there was no leak, because there are only two callers like

    r = deserialize_in_addrs(&lease->dns, dns);

which both keep the unused buffer and later release it.

Note that deserialize_in_addrs() doesn't free the pointer before
reassigning the new output. The caller must take care to to pass
"ret" with an allocated buffer that would be leaked when returning
the result.