[boehm] Use managed allocator only for objects with known jit compile…

… time size.

The allocator always expects to receive two arguments.

mono/metadata/boehm-gc.c

@@ -1062,6 +1062,8 @@ mono_gc_get_managed_allocator (MonoClass *klass, gboolean for_box, gboolean know
 		return NULL;
 	if (klass->byval_arg.type == MONO_TYPE_STRING) {
 		atype = ATYPE_STRING;
+	} else if (!known_instance_size) {
+		return NULL;
 	} else if (!klass->has_references) {
 		if (for_box)
 			atype = ATYPE_FREEPTR_FOR_BOX;