QLinuxFbScreen: fix uninitialized member warnings

Prompted by Coverity CID 11720 (allegedly recently new in dev but
long-standing in 5.6).  Even assuming the class is used correctly -
test initialize() and only use if that succeeded - the destructor
would have passed uninitialized arguments to munmap() and ioctl().

Noticed a double close along the way: it's been fixed on dev but
should have been fixed in 5.6, too.  Documented why ioctl() failure in
switchToGraphicsMode() should at least do no harm.

Change-Id: Ie26a9eefa435b5ff5b1a02e03e29469b8db72d3c
Reviewed-by: Laszlo Agocs <laszlo.agocs@theqtcompany.com>