flex: don't trust YY_USER_INIT

Reported by Bernd Edligner and others.

* src/scan-gram.l: here.

THANKS

@@ -16,6 +16,7 @@ Art Haas                  ahaas@neosoft.com
 Baron Schwartz            baron@sequent.org
 Ben Pfaff                 blp@cs.stanford.edu
 Benoit Perrot             benoit.perrot@epita.fr
+Bernd Edligner            bernd.edlinger@hotmail.de
 Bernd Kiefer              kiefer@dfki.de
 Bert Deknuydt             Bert.Deknuydt@esat.kuleuven.ac.be
 Bill Allombert            Bill.Allombert@math.u-bordeaux1.fr