Fix censoring of Crypto.SHA256.crypt_hash() in Pike/Nettle.

Imported from https://youtrack.roxen.com/issue/PIKE-58

Reported by @grubba

The parameter marked as ****** below is not censored in the thread backtrace:

14:38:41            : >> pike/lib/modules/Nettle.so:1: Crypto.SHA256.crypt_hash("******","sTo30/QP7k99DSc3",UNDEFINED)