Fix incorrect zeroing

clang reports the following warnings:

```
sha2.c:701:32: warning: argument to 'sizeof' in 'memset' call is the same expression as the destination; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
sha2.c:757:31: warning: argument to 'sizeof' in 'memset' call is the same expression as the destination; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
...
```