gammaIncompleteComplInverse not handling p=1 edge case correctly

[tedgin]

std.mathspecial.gammaIncompleteComplInverse(a, p) does not handle the edge case p = 1 correctly. This function computes Q⁻¹(a,p), the inverse of the regularized upper incomplete gamma function, Q(a,x) with respect to x. Here a is positive, and 0 ≤ x ≤ ∞. Q(a,0) = 1 and Q(a,∞) = 0 for all a. For a fixed a ∊ (0,∞), Q(a,x) is a strictly decreasing function of x. Thus when 0 < a < ∞, Q⁻¹(a,1) = 0. However, gammaIncompleteComplInverse(2.0L, 1.0L) evaluates to 7.7307e-12.