[Rd] besselI inaccurate for negative integer order (PR#13556)

[Jerry.Lewis at biogenidec.com]

Full_Name: Jerry W. Lewis
Version: 2.8.1
OS: Windows XP Professional
Submission from: (NULL) (198.180.131.16)


It should be the case that
  besselI(x,-nu) == besselI(x,nu) == besselI(x,abs(nu))
for integer nu, yet R currently can return ridiculous values when nu is a
negative integer.

For instance, besselI(9.6,-44) returns -234626490 instead of the correct value
of 5.9041042646307223e-25, while besselI(9.6,44) gives essentially machine
accuracy.

This is more than an idle mathematical curiosity, since one consequence is that
dskellam in the VGAM package can return values <0 or >1.