Fix misuses of memset

src/md5.c: In function 'sat_MD5_Final':
src/md5.c:269:23: error: argument to 'sizeof' in 'memset' call is the same expression as the destination; did you mean to dereference it? [-Werror=sizeof-pointer-memaccess]
  memset(ctx, 0, sizeof(ctx));
                       ^
examples/solv.c: In function 'read_repos':
examples/solv.c:1616:27: error: argument to 'sizeof' in 'memset' call is the same expression as the destination; did you mean to remove the addressof? [-Werror=sizeof-pointer-memaccess]
     memset(&stb, 0, sizeof(&stb));
                           ^