_solve_..slot_conflicts: fix bug #510270

This fixes an IndexError in _solve_non_slot_operator_slot_conflicts
which occurs when none of the conflict packages matched a particular
atom. This typically means that autounmask broke a USE-dep, but it could
also be due to the slot not matching due to multislot (bug #220341).
Either way, don't try to solve this conflict. Instead, force all of the
associated conflict nodes into the graph so that they are protected from
removal by the conflict solver.

X-Gentoo-Bug: 510270
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=510270