revlog: if the module is initialized more than once, don't leak nullentry

Caught (annoyingly) by the manifest fuzzer.

Differential Revision: https://phab.mercurial-scm.org/D4905

mercurial/cext/revlog.c

@@ -2304,8 +2304,10 b' void revlog_module_init(PyObject *mod)'
 	Py_INCREF(&nodetreeType);
 	PyModule_AddObject(mod, "nodetree", (PyObject *)&nodetreeType);
 
+	if (!nullentry) {
 	nullentry = Py_BuildValue(PY23("iiiiiiis#", "iiiiiiiy#"), 0, 0, 0,
 				  -1, -1, -1, -1, nullid, 20);
+	}
 	if (nullentry)
 		PyObject_GC_UnTrack(nullentry);
 }