gssapi: fix memory leak of output token in multi round context

When multiple rounds are needed to establish a security context
(usually ntlm), we overwrite old token with a new one without free.
Found by proposed gss tests using stub a gss implementation (by
valgrind error), though I have confirmed the leak with a real
gssapi implementation as well.

Closes https://github.com/curl/curl/pull/1733