When there are zero solutions to a multi-solution query, end exploration

[msooseth]

Currently it's not ending exploration, but returns NOTHING, which is terrible, giving WARNING/overapproximation to the user, when in fact the code cannot be explored anyway.

This is a bit of an edge-case but we seem to bump into it quite a bit.

[msooseth]

Ohhh.. see solution in #660 : We should just unwind it all and terminate. I.e. unwind all the frames, and then it will terminate immediately. That is basically a quick way to end exploration.

[gustavo-grieco]

Is this still an issue?

[msooseth]

Well, kinda. We currently:

          0 -> finishFrame (FrameReverted (ConcreteBuf ""))

But that's actually not perfect. Because we are already in a state that a real concrete execution would never be in. So instead we should finishAllFramesAndExit or something.

OK... I did it now, #708