#4826 division by zero computing a Jacobi polynomial

[Barton Willis]

The Jacobi polynomials are defined for all nonnegative integer degrees and all complex arguments. But

(%i2)   jacobi_p(5,-1,1/2,x);
expt: undefined: 0 to a negative exponent.

 -- an error. To debug this try: debugmode(true);

[David Scherfgen]

Fixed by commit [6ac094].