Fix off by one error in out-of-bound check.

Avoid reading pass the end of an array in TTreeFormula.
This fixes http://root.cern.ch/phpBB3/viewtopic.php?t=16969.

(cherry picked from v5-34-00-patch
 commit af5eeb4bdd7ecc59fa8cbb1c6927425a86017a71)