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.
(cherry picked from commit 40ecfad10dd36700028ff0f3d0d79ce7925fe545)