_PyObject_GetDictPtr(): when the offset is negative, always align --

we can't trust that tp_basicsize is aligned.  Fixes SF bug #462848.

Objects/object.c

@@ -1150,20 +1150,15 @@ _PyObject_GetDictPtr(PyObject *obj)
 		return NULL;
 	if (dictoffset < 0) {
 		dictoffset += tp->tp_basicsize;
+		dictoffset += tp->tp_itemsize * ((PyVarObject *)obj)->ob_size;
 		assert(dictoffset > 0); /* Sanity check */
-		if (tp->tp_itemsize > 0) {
-			int n = ((PyVarObject *)obj)->ob_size;
-			if (n > 0) {
-				dictoffset += tp->tp_itemsize * n;
 		/* Round up, if necessary */
-				if (tp->tp_itemsize % PTRSIZE != 0) {
-					dictoffset += PTRSIZE - 1;
+		if (dictoffset % PTRSIZE != 0) {
 			dictoffset /= PTRSIZE;
+			dictoffset += 1;
 			dictoffset *= PTRSIZE;
 		}
 	}
-		}
-	}
 	return (PyObject **) ((char *)obj + dictoffset);
 }