parsers: don't leak a reference to raise_revlog_error on success stable

parsers: don't leak a reference to raise_revlog_error on success

Found with cpychecker.

--- a/mercurial/parsers.c	Fri Jan 23 15:48:18 2015 -0500
+++ b/mercurial/parsers.c	Fri Jan 23 15:50:40 2015 -0500
@@ -1262,6 +1262,7 @@
 			goto classfail;
 		}
 		Py_INCREF(errclass);
+		Py_DECREF(mod);
 	}
 
 	errobj = PyObject_CallFunction(errclass, NULL);