mame: finally make mem reading work with frame advance

proof that it's a good idea to go to bed with a problem, because your brain keeps solving it while you're sleeping. so when you wake up, it presents you the solution that actually works.

src/BizHawk.Emulation.Cores/Arcades/MAME/MAME.cs

@@ -155,6 +155,11 @@ namespace BizHawk.Emulation.Cores.Arcades.MAME
 			_paused = false;
 			_frameDone = false;
 
+			if (_memAccess)
+			{
+				_mamePeriodicComplete.WaitOne();
+			}
+
 			for (; _frameDone == false;)
 			{
 				_mameFrameComplete.WaitOne();
@@ -638,7 +643,7 @@ namespace BizHawk.Emulation.Cores.Arcades.MAME
 				if (!_frameDone && !_paused)
 				{
 					_memAccess = false;
-					break;
+					return;
 				}
 			}