No need for strncpy in passwordCallback

No need for strncpy in passwordCallback

Summary:
[Folly] No need for `strncpy` in `passwordCallback`.

Careful reading of the documentation:

> The pem_passwd_cb must write the password into the provided buffer `buf` which is of size `size`.
>
> https://wiki.openssl.org/index.php?title=Manual:SSL_CTX_set_default_passwd_cb(3)&oldid=761

No mention is made of a requirement on the password being written into `buf` that it be null-terminated.

Reviewed By: knekritz, meyering

Differential Revision: D5524814

fbshipit-source-id: 6cfc588cdf3675281ffe39e6af376f3f0631d1b0