Fix debugger throwing exceptions on opened on a core that does not implement IDisassemblable

BizHawk.Client.EmuHawk/tools/Debugger/GenericDebugger.Disassembler.cs

@@ -37,7 +37,10 @@ namespace BizHawk.Client.EmuHawk
 
 		public void UpdatePC()
 		{
-			currentDisassemblerAddress = (uint)PCRegister.Value;
+			if (CanDisassemble)
+			{
+				currentDisassemblerAddress = (uint)PCRegister.Value;
+			}
 		}
 
 		private void UpdateDisassembler()

BizHawk.Client.EmuHawk/tools/Debugger/GenericDebugger.IToolForm.cs

@@ -28,6 +28,7 @@ namespace BizHawk.Client.EmuHawk
 
 		#region Implementation checking
 
+		// TODO: be cachey with checks that depend on catching exceptions
 		private bool CanUseMemoryCallbacks
 		{
 			get