Fix an assertion in polybori

Imported from <a href="http://trac.sagemath.org/sage_trac/ticket/13883">trac #13883</a>.

patch for pbori.pyx

Reported by: SimonKing

Component: commutative algebra

CC: AlexanderDreyer,, PolyBoRi

Report Upstream: Reported upstream. No feedback yet.

Authors: Alexander Dreyer

Dependencies: 

Work issues: 

Reviewers: Jean-Pierre Flori

Merged in: sage-5.6.beta3

Attachments: <ol><li><a href="http://trac.sagemath.org/sage_trac/attachment/ticket/13883/FGLMStrategy_sigon.patch">FGLMStrategy_sigon.patch: patch for pbori.pyx</a> by AlexanderDreyer at 20130102T00:15:46</li></ol>