instead of passing '0' in the initializer list to do aggregate
initialization, just use zero initialization. simpler this way.
also, this helps to silence a `-Wmissing-braces` warning, like
```
/home/kefu/dev/scylladb/auth/passwords.cc:21:43: error: suggest braces around initialization of subobject [-Werror,-Wmissing-braces]
static thread_local crypt_data tlcrypt = {0, };
^
{}
```
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
Closes#13060
ac575d0b0e