Win32: Don't hook assertion handler when debugging

Sven2 reports he can't get reliable stack traces from his debugger when
the assertion handler is installed. Since there's no need for the hook
when we're already running under a debugger, don't install it.

src/platform/C4CrashHandlerWin32.cpp

@@ -637,8 +637,9 @@ void InstallCrashHandler()
 	SetUnhandledExceptionFilter(GenerateDump);
 
 #ifndef NDEBUG
-	// Hook _wassert/_assert
-	HookAssert(&assertion_handler);
+	// Hook _wassert/_assert, unless we're running under a debugger
+	if (!IsDebuggerPresent())
+		HookAssert(&assertion_handler);
 #endif
 }