sha: fix use-after-free issue

[qmuntal]

There is a possible use-after-free issue in evpHash.WriteString and in evpHash.WriteByte. They both pass h.ctx to EVP_DigestUpdate without making sure h is still alive until the cgo call ends, i.e. using runtime.KeepAlive(h).

If the garbage collector chimes in at the right time, it will collect h, calling it's finalizer, that free's h.ctx, likely producing an invalid access exception.

Ported from microsoft/go-crypto-openssl#55.