Calling GetConsoleCP() when ReadConsoleW() fails (potentially indicating
that cmd.exe's instance isn't attached to a console) makes no sense.

In details, since GetConsoleCP() returns 0 when not attached to a console
to indicate error, we're in fact using CP_ACP.

So fallback explicitely to OEM CP (which is the default for CUI programs).
Signed-off-by: Eric Pouech <eric.pouech@gmail.com>