Merge pull request #1222 from Unity-Technologies/unity-master-debugger-fix-create-boxed-value

Fixing issue with debugger CREATE_BOXED_VALUE command. case 1169306.

Author: joncham

mono/mini/debugger-agent.c

@@ -9539,6 +9539,9 @@ domain_commands (int command, guint8 *p, guint8 *end, Buffer *buf)
 		// FIXME:
 		g_assert (domain == domain2);
 
+		if (!klass->valuetype)
+			return ERR_INVALID_ARGUMENT;
+
 		o = mono_object_new_checked (domain, klass, &error);
 		mono_error_assert_ok (&error);