socket: fix use of ERRNO_IS_DISCONNECT()

socket: fix use of ERRNO_IS_DISCONNECT()

Given that ERRNO_IS_DISCONNECT() also matches positive values,
make sure this macro is not called with arguments that do not have
errno semantics.

In this case the argument passed to ERRNO_IS_DISCONNECT() is the value
returned by socket_acquire_peer() which can legitimately return 1
without errno semantics, so fix this by moving ERRNO_IS_DISCONNECT()
invocation to the branch where the return value is known to be negative.