[mono][interp] Add unbox when calling valuetype method through delegate

If we are calling an open instance delegate, where the target method is on a valuetype, we will need to unbox this pointer.

Author: BrzVlad

src/mono/mono/mini/interp/interp.c

@@ -3975,7 +3975,12 @@ mono_interp_exec_method (InterpFrame *frame, ThreadContext *context, FrameClause
 					} else if ((m_method_is_virtual (del_imethod->method) && !m_method_is_static (del_imethod->method)) && !del->target && !m_class_is_valuetype (del_imethod->method->klass)) {
 						// 'this' is passed dynamically, we need to recompute the target method
 						// with each call
-						del_imethod = get_virtual_method (del_imethod, LOCAL_VAR (call_args_offset + MINT_STACK_SLOT_SIZE, MonoObject*)->vtable);
+						MonoObject *obj = LOCAL_VAR (call_args_offset + MINT_STACK_SLOT_SIZE, MonoObject*);
+						del_imethod = get_virtual_method (del_imethod, obj->vtable);
+						if (m_class_is_valuetype (del_imethod->method->klass)) {
+							// We are calling into a value type method, `this` needs to be unboxed
+							LOCAL_VAR (call_args_offset + MINT_STACK_SLOT_SIZE, gpointer) = mono_object_unbox_internal (obj);
+						}
 					} else {
 						del->interp_invoke_impl = del_imethod;
 					}