iterating gi.invent (github issue #1315)

GitHub issue #1315 points out that it is possible for
a downstream function to change an object's nobj field
to point to a completely different chain.

The cited example by @vultur-cadens was:

     for (obj = gi.invent; obj; obj = obj->nobj)
         if (obj->oclass != COIN_CLASS && !obj->cursed && !rn2(5)) {
             curse(obj);
             ++buc_changed;
         }

    curse() drops the weapon with drop_uswapwep(),
        which calls dropx(),
            which calls dropy(),
                which calls dropz(),
                    which calls place_object().

place_object alters the nobj pointer, to point to the floor chain:
    otmp->nobj = fobj;
    fobj = otmp;

The result was that the next loop iteration was then using floor
objects from the floor chain.

This alters several for-loops to use a more consistent approach,
particularly when the obj is being handed off to a function,
where a downstream function might, or might not, alter the nobj
field.

References:

https://github.com/NetHack/NetHack/issues/1315
https://www.reddit.com/r/nethack/comments/1gkc9ub/even_if_you_drop_an_item_before_drinking_from_the/

src/mhitu.c

@@ -1252,7 +1252,7 @@ gulpmu(struct monst *mtmp, struct attack *mattk)
     struct trap *t = t_at(u.ux, u.uy);
     int tmp = d((int) mattk->damn, (int) mattk->damd);
     int tim_tmp;
-    struct obj *otmp2;
+    struct obj *otmp2, *nextobj;
     int i;
     boolean physical_damage = FALSE;
 
@@ -1357,8 +1357,10 @@ gulpmu(struct monst *mtmp, struct attack *mattk)
         u.uswldtim = (unsigned) ((tim_tmp < 2) ? 2 : tim_tmp);
         swallowed(1); /* update the map display, shows hero swallowed */
         if (!flaming(mtmp->data)) {
-            for (otmp2 = gi.invent; otmp2; otmp2 = otmp2->nobj)
+            for (otmp2 = gi.invent; otmp2; otmp2 = nextobj) {
+                nextobj = otmp2->nobj;
                 (void) snuff_lit(otmp2);
+            }
         }
     }