In a machine where INT64_IS_BUSTED, we can only support 32-bit values

In a machine where INT64_IS_BUSTED, we can only support 32-bit values
for int8 and related types.  However we might be talking to a client
that has working int64; so pq_getmsgint64 really needs to check the
incoming value and throw an overflow error if we can't represent it
accurately.