sd-resolve: fix check for packet size

sd-resolve: fix check for packet size

The protocol is that a string is serialized with the nul byte at the end, and
the terminator is included in length. We'd call strndup with offset 0, length
len1-1, and then a second time with offset len1, length len2-1, so in the end
the check was off by one. But let's require the terminating nul too, even if
we don't access it.

CID #1383035.