authenticate_gss_server_step: fix memory leaks

[brianbao]

In authenticate_gss_server_step there are two memory leaks:

• target_name gets malloc'd by gss_inquire_context, but never gets free'd by gss_release_name. See targ_name here.
• output_token gets malloc'd in gss_display_name on line 379, but isn't free'd by gss_release_buffer before reuse on line 403. See output_name_buffer here.

I ran some tests internally using valgrind to verify. Here was the output:

Before:

==1014051== LEAK SUMMARY:
==1014051==    definitely lost: 61,000 bytes in 2,000 blocks
==1014051==    indirectly lost: 288,932 bytes in 9,997 blocks
==1014051==      possibly lost: 5,168 bytes in 17 blocks
==1014051==    still reachable: 3,929 bytes in 32 blocks
==1014051==         suppressed: 0 bytes in 0 blocks

After:

==1105036== LEAK SUMMARY:
==1105036==    definitely lost: 0 bytes in 0 blocks
==1105036==    indirectly lost: 0 bytes in 0 blocks
==1105036==      possibly lost: 4,256 bytes in 14 blocks
==1105036==    still reachable: 3,861 bytes in 29 blocks
==1105036==         suppressed: 0 bytes in 0 blocks

I also made a minor update to propagate the output token if gss_accept_sec_context errors.