check for very large or infinity float64 in the alpha parameter to prevent an infinite loop

[Mrucznik]

Hello
There is an infinite loop in the gamma function. Reproduction:

func TestInfinityGamma(t *testing.T) {
	g := NewGammaGenerator(0)

	fmt.Println(g.Gamma(math.MaxFloat64/2, 1)) // this will be ok

	fmt.Println(g.Gamma(math.MaxFloat64, 1)) // infinite loop
	fmt.Println(g.Gamma(math.Inf(1), 1))     // infinite loop
}

I added a check for values bigger than math.MaxFloat64/2, to prevent this.