MD5: fix bug where ctx is not properly zeroed
MD5: fix bug where ctx is not properly zeroed

Adapted from MySQL's identical patch:
http://bugs.mysql.com/bug.php?id=68909

The memset was being passed a `len` of the size of the pointer,
instead of the size of the pointer's contents.

mvtiaine/uade,mvtiaine/uade,mvtiaine/uade,mvtiaine/uade