[v9_16] Free resources when gss_accept_sec_context() fails

Even if a call to gss_accept_sec_context() fails, it might still cause a GSS-API response token to be allocated and left for the caller to release. Make sure the token is released before an early return from dst_gssapi_acceptctx().

(cherry picked from commit d954e152)

Closes #2620 (closed)