bugfix(gui): Fix crash when RenderFpsFontSize is set to 0

[L3-M]

• Follow up for feat(ui): Implement simple Latency and FPS display #1546

This change fixes a crash when RenderFpsFontSize = 0 in Generals and Zero Hour.

[xezon]

What was the crash call stack?

[L3-M]

What was the crash call stack?

 >	generalszh.exe!FontCharsClass::Store_GDI_Char(wchar_t ch) Line 1391	C++
 	generalszh.exe!FontCharsClass::Get_Char_Data(wchar_t ch) Line 1256	C++
 	generalszh.exe!FontCharsClass::Get_Char_Spacing(wchar_t ch) Line 1289	C++
 	generalszh.exe!Render2DSentenceClass::Allocate_New_Surface(const wchar_t * text, bool justCalcExtents) Line 645	C++
 	generalszh.exe!Render2DSentenceClass::Build_Sentence_Not_Centered(const wchar_t * text, int * hkX, int * hkY, bool justCalcExtents) Line 1005	C++
 	generalszh.exe!Render2DSentenceClass::Get_Formatted_Text_Extents(const wchar_t * text) Line 289	C++
 	generalszh.exe!W3DDisplayString::computeExtents() Line 362	C++
 	generalszh.exe!W3DDisplayString::setFont(GameFont * font) Line 308	C++
 	generalszh.exe!InGameUI::refreshRenderFpsResources() Line 5947	C++
 	generalszh.exe!InGameUI::refreshCustomUiResources() Line 5909	C++
 	generalszh.exe!InGameUI::reset() Line 2071	C++
 	generalszh.exe!W3DInGameUI::reset() Line 381	C++
 	generalszh.exe!GameClient::reset() Line 457	C++
 	generalszh.exe!W3DGameClient::reset() Line 106	C++
 	generalszh.exe!SubsystemInterfaceList::resetAll() Line 183	C++
 	generalszh.exe!GameEngine::resetSubsystems() Line 812	C++
 	generalszh.exe!GameEngine::init() Line 769	C++
 	generalszh.exe!Win32GameEngine::init() Line 66	C++
 	generalszh.exe!GameMain() Line 46	C++
 	generalszh.exe!WinMain(HINSTANCE__ * hInstance, HINSTANCE__ * hPrevInstance, char * lpCmdLine, int nCmdShow) Line 903	C++
 	[External Code]	
 	[Frames below may be incorrect and/or missing, no symbols loaded for kernel32.dll]	

[xezon]

Ok. We probably want to make another change where Store_GDI_Char does not crash on 0 font, but raises Assert instead.