SHA2 segmentation fault

seems to be caused by  721fb99e

```nim
import nimcrypto

var hctx = default(sha_256)
var s = newSeq[byte](133)
hctx.update(addr s[0], 133)
```

```
/home/ryan/dev/nim/clones/nimcrypto/nimcrypto/sha2/sha2.nim(284) update
/home/ryan/dev/nim/clones/nimcrypto/nimcrypto/sha2/sha2.nim(267) update
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
```
