The pull request by argrath changes weight_cap() to never return a
value less than 1 because try_lift() divides by that return value
and a 0 would trigger a crash. The code involved is used when
attempting to pull a monster out of a pit via #untrap.
I'm fairly sure that weight_cap() can never produce a value that's
less than 1 already, but have put in a variation of the PR's fix.
I've also implemented a different fix that removes the division
from try_lift(). The original code seems to have gone out of its
way to avoid calculating inv_weight() twice, but doing the latter
(for the once in a hundred games where it might happen) greatly
simplifies things by removing details of carrying capacity.
Fixes#621
b4908f6928