In init_ctx_call_init(), if gss_init_sec_context() fails while
producing the first SPNEGO initiator token, we remove the first
candidate mechanism from sc->mech_set and try again. If
sc->ctx_handle is present after the error (more likely after commit
56f7b1bc95a2a3eeb420e069e7655fb181ade5cf), we must clear it before
falling back or it will cause subsequent attempts to fail.