The code path for SPNEGO_FALLBACK mixed up the order of
memory context, event context. As one is "void *" and the other "struct tevent_context *" the compiler doesn't complain about it.

-Wc++-compat would warn about it:
warning: request for implicit conversion from ‘TALLOC_CTX *’ to ‘struct tevent_context *’ not permitted in C++ [-Wc++-compat]

But it also warns about a lot of other stuff, so it's not easy to
add it without -Wno-error=c++-compat.