trivial bug: sign change for negative values in bessel_i1

[Cornelia Strauß <strauss@…>]

The function bessel_i1() in boost/math/special_functions/detail/bessel_i1.hpp ends with the following code:

if (x < 0)
{
    value *= -value;                 // odd function
}
return value;

Instead of taking the square root, only the sign of value should be changed:

if (x < 0)
{
    value *= -1;                 // odd function
}
return value;

[John Maddock]

Some investigation shows that that is dead code: negative x is handled by the caller of that function. I've removed the offending code and added an assert for x >= 0 instead. Fixed in Git develop.