Fix various bugs and errors

[EXL]

1. Fix bug with wrong value in the "Page Skin" combo box ("Settings => Style" dialog).
   Steps to reproduce:
   1.1. Run new and clear instance of the Cool Reader, go to the "Settings => Style => Page Skin".
   1.2. It shows bg_paperX (instead of [NONE]).
   1.3. Choose second (after [NONE]) page skin, click "OK".
   1.4. Nothing will change.

2. Fix compilation error on the new GCC versions:

/home/exl/Projects/GIT/coolreader/cr3qt/src/cr3widget.cpp: In constructor ‘CR3View::CR3View(QWidget*)’:
/home/exl/Projects/GIT/coolreader/cr3qt/src/cr3widget.cpp:325:47: error: cannot bind non-const lvalue reference of type ‘LVRefVec<LVImageSource>&’ to an rvalue of type ‘LVRefVec<LVImageSource>’
     _docview->setBatteryIcons( getBatteryIcons(0x000000) );
                                ~~~~~~~~~~~~~~~^~~~~~~~~~
In file included from /home/exl/Projects/GIT/coolreader/cr3qt/src/cr3widget.cpp:1:0:
/home/exl/Projects/GIT/coolreader/tinydict/../crengine/include/lvdocview.h:608:10: note:   initializing argument 1 of ‘void LVDocView::setBatteryIcons(LVRefVec<LVImageSource>&)’
     void setBatteryIcons( LVRefVec<LVImageSource> & icons );
          ^~~~~~~~~~~~~~~

3. Fix Segmentation fault in the Release build:

$ gdb ./cr3 
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Reading symbols from ./cr3...(no debugging symbols found)...done.
(gdb) r
Starting program: /home/exl/Projects/GIT/coolreader/qtbuild/cr3qt/cr3 
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/usr/lib/libthread_db.so.1".
2018/05/03 11:32:41.5931 WARN Changing log level from 3 to 1
642 fonts loaded.
2018/05/03 11:32:41.8991 ERROR Canot load translation file cr3_en_US from dir /usr/share/cr3/i18n/
2018/05/03 11:32:41.9328 ERROR Loading settings from file /home/exl/.cr3/cr3.ini

Program received signal SIGSEGV, Segmentation fault.
0x000055555567bc28 in LVStyleSheet::apply(ldomNode const*, css_style_rec_tag*) ()
(gdb) bt full
#0  0x000055555567bc28 in LVStyleSheet::apply(ldomNode const*, css_style_rec_tag*) ()
#1  0x00005555556d2d23 in setNodeStyle(ldomNode*, LVFastRef<css_style_rec_tag>, LVProtectedFastRef<LVFont>) ()
#2  0x0000555555640954 in ldomNode::initNodeStyle() ()
#3  0x0000555555640e50 in updateStyleDataRecursive(ldomNode*) ()
#4  0x0000555555640ef0 in updateStyleDataRecursive(ldomNode*) ()
#5  0x0000555555640ef0 in updateStyleDataRecursive(ldomNode*) ()
#6  0x0000555555640ef0 in updateStyleDataRecursive(ldomNode*) ()
#7  0x0000555555646771 in ldomDocument::render(LVRendPageList*, LVDocViewCallback*, int, int, bool, int, LVProtectedFastRef<LVFont>, int, LVFastRef<CRPropAccessor>) ()
#8  0x00005555556ae862 in LVDocView::Render(int, int, LVRendPageList*) ()
#9  0x00005555556aea50 in LVDocView::checkRender() ()
#10 0x00005555556b2fa6 in LVDocView::updateBookMarksRanges() ()
#11 0x00005555556c7815 in LVDocView::propsApply(LVFastRef<CRPropAccessor>) ()
#12 0x00005555555ce64e in CR3View::loadSettings(QString) ()
#13 0x00005555555da585 in MainWindow::MainWindow(QWidget*) ()
#14 0x00005555555b928c in main ()

[Frenzie]

Also see koreader/crengine#90 (comment)

Just my 2 cents on it. @poire-z You shouldn't compare this with NULL/nullptr. It's invalid C++ with undefined behavior. It means it might work with one compiler version but won't with another compiler/version (can be optimized out as noop).
So all null reference checks should be done on instance and not inside instance method.

You can read more about this in the following discussion:
https://www.reddit.com/r/cpp_questions/comments/3roy3e/this_nullptr_can_this_ever_be_true/

[…]

if (!node) Yeah. It's a C style which still works. But it's not very welcome now in cpp. It's better to distinguish null checks from bool checks and use: if(node == NULL)
Though it doesn't really matter at least for now.