Fix runtime crash in OpenSSL with "Illegal instruction" by making some
casts a bit less evil.

This was e.g. seen when using portsnap as:

Fetching snapshot tag from portsnap3.FreeBSD.org... Illegal instruction

Note the patch is slightly different from kan's original patch to
match style in the OpenSSL source files a bit better.

Submitted by: kan
Tested by: many