Minor changes to avoid undefined behaviour warnings.

Avoid creating a reference from a nullptr.
Avoid trying a malloc and memcpy into a 0 length buffer.
Virtual trains should never be drawn.

src/pathfinder/pf_performance_timer.hpp

@@ -51,7 +51,7 @@ struct CPerfStartReal
 {
 	CPerformanceTimer *m_pperf;
 
-	inline CPerfStartReal(CPerformanceTimer& perf) : m_pperf(&perf)
+	inline CPerfStartReal(CPerformanceTimer* perf) : m_pperf(perf)
 	{
 		if (m_pperf != NULL) m_pperf->Start();
 	}
@@ -72,7 +72,7 @@ struct CPerfStartReal
 
 struct CPerfStartFake
 {
-	inline CPerfStartFake(CPerformanceTimer& perf) {}
+	inline CPerfStartFake(CPerformanceTimer* perf) {}
 	inline ~CPerfStartFake() {}
 	inline void Stop() {}
 };