mcookie: correct wrong memset argument

mcookie: correct wrong memset argument

Fixes #10216

Building mcookie generates a warning about possible wrong arguments to
memset:

mcookie.c:207:26: warning: argument to ‘sizeof’ in ‘memset’ call is the same expression
  as the destination; did you mean to dereference it? [-Wsizeof-pointer-memaccess]
     memset(ctx, 0, sizeof(ctx)); /* In case it's sensitive */

ctx is a pointer to a structure, so the code should use the size of the
structure and not the size of the pointer when it tries to clear the
structure, similar to how it got fixed upstream back in 2009:

https://git.kernel.org/pub/scm/utils/util-linux/util-linux.git/commit/lib/md5.c?id=6596057175c6ed342dc20e85eae8a42eb29b629f

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>