sd-varlink: fix a potential connection count leak

sd-varlink: fix a potential connection count leak

With the old version there was a potential connection count leak if
either of the two hashmap operations in count_connection() failed. In
that case we'd return from sd_varlink_server_add_connection_pair()
_before_ attached the sd_varlink_server object to an sd_varlink object,
and since varlink_detach_server() is the only place where the connection
counter is decremented (called through sd_varlink_close() in various
error paths later _if_ the "server" object is not null, i.e. attached to
the sd_varlink object) we'd "leak" a connection every time this
happened. However, the potential of abusing this is very theoretical,
as one would need to hit OOM every time either of the hashmap operations
was executed for a while before exhausting the connection limit.

Let's just increment the connection counter after any potential error
path, so we don't have to deal with potential rollbacks.