Fix memset size argument error.

Detected by gcc:

  md5.c: In function ‘MD5Final’:
  md5.c:245:26: warning: argument to ‘sizeof’ in ‘memset’ call is the same expression as the destination; did you mean to dereference it? [-Wsizeof-pointer-memaccess]

Signed-off-by: Michael Buesch <m@bues.ch>