qga: fix off-by-one length check

Laszlo Ersek said: "The length check is off by one (in the safe direction); it
should be (nchars >= 2). The processing should be active for the wide string
L"\r\n" -- resulting in the empty wide string --, I believe."

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
