Fix rc4 string-to-key on unterminated inputs

Fix rc4 string-to-key on unterminated inputs

The internal UTF-8 to UCS-2 conversion functions did not properly
respect their length arguments, instead assuming that the input string
is terminated with a zero bytes.  As a result,
krb5int_arcfour_string_to_key could fail on unterminated inputs.  Fix
the underlying support functions to read their inputs only up to the
specified length.

ticket: 7643 (new)