Not necessarily relevant outside windows, where the socket descriptor
is SOCKET (= unsigned 64-bit). Also follow their recommendation to not
compare to -1, but rather to INVALID_SOCKET.

Pick-to: 6.4 6.3 6.2
Change-Id: I0cfa4dfd9e147469132e2e72de22b30eab01e15c
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>