math/big: correct quadratic space complexity in Mul.

The previous implementation used to have a O(n) recursion
depth for unbalanced inputs. A test is added to check that a
reasonable amount of bytes is allocated in this case.

Fixes #3807.

R=golang-dev, dsymonds, gri
CC=golang-dev, remy
https://golang.org/cl/6345075
2 files changed