Py_DECREF: only pass filename if Py_REF_DEBUG is defined (GH-17870)

Filename and line numbers are not needed when Py_REF_DEBUG are not
defined.

The static inline _Py_DECREF() function was introduced by
commit 2aaf0c12041bcaadd7f2cc5a54450eefd7a6ff12.
1 file changed