Redo the level if stairs cannot be placed

[flend]

Rapid Brogue seed 519 cannot place stairs in level 2 since there are no qualifying squares (basically every square is part of a machine). This patch rerolls the level when this (very rare) case occurs.

[nstoddard]

Does this fix #536 too?

[flend]

Does this fix #536 too?

Yes, I didn't know the issue existed but I recreated the crash on v1.13 release and this patch fixes it.
The discussion in the ticket is interesting and you could make the machine-placing code more complex to avoid eating up all the level but I think this solution should be added in any case: it's more general and in-line with usual roguelike procgen practices.