Do not invoke UB in BigInt shr operations

Shifting a value of type T by an amount that's greater or equal to the
size of the type itself is UB.

Spotted by @tgschultz
9ec4ccc68fLemonBoy committed on 4/26/2019, 6:24:35 PM· committed by Andrew Kelleyparent4df2f3d
2 files changedLine totals unavailable