Use PyMethod_GET_SELF instead access the PyMethodObject field directly

Pyston use PyMethodObject as an opaque pointer. So we should not access its
member directly.

include/ExtensionClass/ExtensionClass.h

@@ -208,8 +208,10 @@ static PyExtensionClass NAME ## Type = { PyObject_HEAD_INIT(NULL) 0, # NAME, \
   PyExtensionClassCAPI->PyECMethod_New_((CALLABLE),(INST))
 
 /* Return the instance that is bound by an extension class method. */
+// Pyston change:
 #define PyECMethod_Self(M) \
-(PyMethod_Check((M)) ? ((PyMethodObject*)(M))->im_self : NULL)
+(PyMethod_Check((M)) ? PyMethod_GET_SELF(M) : NULL)
+// (PyMethod_Check((M)) ? ((PyMethodObject*)(M))->im_self : NULL)
 
 /* Check whether an object has an __of__ method for returning itself
    in the context of it's container. */