s3:lib: Use memcpy() in escape_ldap_string()

s3:lib: Use memcpy() in escape_ldap_string()

../source3/lib/ldap_escape.c: In function ‘escape_ldap_string’:
../source3/lib/ldap_escape.c:79:4: error: ‘strncpy’ output truncated
    before terminating nul copying 3 bytes from a string of the same length
[-Werror=stringop-truncation]
    strncpy (p, sub, 3);
    ^~~~~~~~~~~~~~~~~~~

We concatenat and do not care about NUL-termination till the loop has
finished.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13437

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>