fix pull request #621 - potential divide by 0

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

src/hack.c

@@ -3330,8 +3330,6 @@ weight_cap(void)
             if (EWounded_legs & RIGHT_SIDE)
                 carrcap -= 100;
         }
-        if (carrcap < 0)
-            carrcap = 0;
     }
 
     if (ELevitation != save_ELev || BLevitation != save_BLev) {
@@ -3340,7 +3338,7 @@ weight_cap(void)
         float_vs_flight();
     }
 
-    return (int) carrcap;
+    return (int) max(carrcap, 1L); /* never return 0 */
 }
 
 /* returns how far beyond the normal capacity the player is currently. */