sd-bus: don't look for a 64bit value when we only have 32bit value on reply cookie...

This broke hashtable lookups for the message cookies on s390x, which is
a 64bit BE machine where accessing 32bit values as 64bit and vice versa
will explode.

Also, while we are at it, be a bit more careful when dealing with the
64bit cookies we expose and the 32bit serial numbers dbus uses in its
payload.

Problem identified by Fridrich Strba.