This avoids that the SHA256 implementation would produce wrong paddings
and thus wrong digests for messages of length >= 2^32 bytes on 32-bit
platforms.

This is not exploitable in any way since the SHA256 API is an internal
API and we never call it with that long messages.

Since missing elements are initialized with zeros, this change is
purely syntactical.

The vector has been taken from https://www.di-mgt.com.au/sha_testvectors.html.
It can be independently verified using the following Python code.

```
h = hashlib.sha256()
for i in range(1_000_000):
    h.update(b'a')
print(h.hexdigest())
```