Fixed the IndexError bug for matroids of size >= 11
Trac #16623: Fix bugs in plot() and show() methods of abstract matroids class

The following produces {{{IndexError}}}

{{{
matroids.Uniform(3,14).show()
}}}

URL: http://trac.sagemath.org/16623
Reported by: Jayant
Ticket author(s): Jayant Apte
Reviewer(s): Stefan van Zwam