s3-librpc-crypto: avoid crash with MIT krb5 1.10.0 in gss_get_name_attribute()

s3-librpc-crypto: avoid crash with MIT krb5 1.10.0 in gss_get_name_attribute()

gss_get_name_attribute() can return unintialized pac_display_buffer
and later gss_release_buffer() will crash on attempting to release it.

The fix on MIT krb5 side is in 1.10.1, reported in both Debian and MIT upstream:
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=658514
http://krbdev.mit.edu/rt/Ticket/Display.html?user=guest&pass=guest&id=7087

We need to initialize variables before using gss_get_name_attribute()

Fix bug #8988 (avoid crash with MIT krb5 1.10.0 in gss_get_name_attribute()).