avoid lava recursion

avoid lava recursion

     Noticed while working on the "lava inconsistencies" patch:  it was
possible for lava_effects() to be called recursively, despite a patch
five and half years ago which was supposed to prevent that.  That patch
handled the case where the hero dies and gets life-saved, but it didn't
deal with a hero who survives due to water walking boots and then has
those burned away.  [Not an issue in 3.4.3 because Boots_off() didn't
call spoteffects() for lava then, but it does in post-3.4.3 branch as
well as trunk.  fixes34.4 has an entry about fixing an "object lost"
panic for this but I don't think it's accurate.  I think that the panic
only became possible after the post-3.4.3 "handle lava when removing or
losing water walking boots" fix was introduced.]