Don't crash if FontManager::getByName returns NULL (Fixes #3552)

apps/openmw/mwgui/bookpage.cpp

@@ -275,9 +275,12 @@ struct TypesetBookImpl::Typesetter : BookTypesetter
             if (i->match (fontName, fontColour, fontColour, fontColour, 0))
                 return &*i;
 
-        StyleImpl & style = *mBook->mStyles.insert (mBook->mStyles.end (), StyleImpl ());
+        MyGUI::IFont* font = MyGUI::FontManager::getInstance().getByName(fontName);
+        if (!font)
+            throw std::runtime_error(std::string("can't find font ") + fontName);
 
-        style.mFont = MyGUI::FontManager::getInstance().getByName(fontName);
+        StyleImpl & style = *mBook->mStyles.insert (mBook->mStyles.end (), StyleImpl ());
+        style.mFont = font;
         style.mHotColour = fontColour;
         style.mActiveColour = fontColour;
         style.mNormalColour = fontColour;

apps/openmw/mwgui/formatting.cpp

@@ -413,7 +413,10 @@ namespace MWGui
         int TextElement::currentFontHeight() const
         {
             std::string fontName(mTextStyle.mFont == "Default" ? MyGUI::FontManager::getInstance().getDefaultFont() : mTextStyle.mFont);
-            return MyGUI::FontManager::getInstance().getByName(fontName)->getDefaultHeight();
+            MyGUI::IFont* font = MyGUI::FontManager::getInstance().getByName(fontName);
+            if (!font)
+                return 0;
+            return font->getDefaultHeight();
         }
 
         int TextElement::getHeight()