Various changes suggested by cppcheck

- remove unused variables
- reduce the scope where it makes sense
- correct limits (did you know that strcat()'s last parameter does not
  include the \0 that is always added?)
- set some free()'d pointers to NULL

Source/Core/VideoCommon/Fifo.cpp

@@ -70,6 +70,7 @@ void Fifo_Shutdown()
 {
 	if (GpuRunningState) PanicAlert("Fifo shutting down while active");
 	FreeMemoryPages(videoBuffer, FIFO_SIZE);
+	videoBuffer = NULL;
 }
 
 u8* GetVideoBufferStartPtr()