Fixed gcc warning for the md5 driver.

```
md5.c: In function 'MD5Final':
md5.c:166:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
md5.c:167:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
```
Debian Gnu/Linux on i386 build against 2.0.4 :

https://buildd.debian.org/status/fetch.php?pkg=fwknop&arch=i386&ver=2.0.4-1&stamp=1358610541