Allow null outputs to gss_get_name_attribute()

Allow null outputs to gss_get_name_attribute()

In krb5_gss_get_name_attribute(), always ask for kvalue and
kdisplay_value when calling krb5_authdata_get_attribute(), as it
currently expect non-null arguments.  This change allows applications
to pass GSS_C_NO_BUFFER for the value and display_value output
parameters.  (Passing NULL for the authenticated and complete output
parameters already works.)

[ghudson@mit.edu: initialized kvalue and kdisplay_value for safety]

ticket: 8557 (new)
target_version: 1.15-next
target_version: 1.14-next
tags: pullup