BUG: fix undefined behavior in alngam

INT(A) is undefined if A cannot be represented as INT
In MIPS the result is positive triggering a huge loop for:

    scipy.stats.skellam.sf(np.inf, 10, 11)
Merge pull request #3899 from juliantaylor/skellam-ub

BUG: special: fix undefined behavior in alngam

INT(A) is undefined if A cannot be represented as INT
In MIPS the result is positive triggering a huge loop.