PyUno fixed crash in function isInstanceOfStructOrException()

when we try to call PyUNO_callable object that doesn't have __class__ attribute

Change-Id: Ia05f70d70f248d50aa141b09625f7ec50189e1dd
Reviewed-on: https://gerrit.libreoffice.org/4309Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>

pyuno/source/module/pyuno_except.cxx

@@ -200,8 +200,10 @@ bool isInstanceOfStructOrException( PyObject *obj)
     PyRef attr(
         PyObject_GetAttrString(obj, "__class__"),
         SAL_NO_ACQUIRE );
-    return PyObject_HasAttrString(
-        attr.get(), "__pyunostruct__");
+    if(attr.is())
+        return PyObject_HasAttrString(attr.get(), "__pyunostruct__");
+    else
+        return false;
 }
 
 sal_Bool isInterfaceClass( const Runtime &runtime, PyObject * obj )