Make RC4 string-to-key more robust

Make RC4 string-to-key more robust

krb5int_utf8cs_to_ucs2les() can read slightly beyond the end of the
input buffer if the buffer ends with an invalid UTF-8 sequence.  When
computing the RC4 string-to-key result, make a zero-terminated copy of
the input string and use krb5int_utf8s_to_ucs2les() instead.

ticket: 8576 (new)
target_version: 1.15-next
target_version: 1.14-next
tags: pullup