R888 - panic while quaffing a potion of levitation

Now that the in_use flag is set for potions being quaffed, use the in_use
flag in general in destroy_item to avoid destroying the in use object, on
the assumption that the caller will call useup when finished.  There are a
few places that set then unset in_use, but these don't currently result in
a call to destroy_item.  The current_wand hack was not removed, since its
logic appears to allow destroy_item to still destroy the item.

doc/fixes34.1

@@ -159,6 +159,8 @@ blessed gain level when already at level 30 won't reduce experience points
 keep counting spell skill exercise even after expert status is reached
 when a fountain dries up or a throne vanishes, make sure it really happens
 allow player to name polymorph potion if nothing seems to happen
+avoid crash when drinking a potion causes the hero to float up over a fire
+	trap, for example, which might try to destroy the in-use potion
 
 
 Platform- and/or Interface-Specific Fixes

src/zap.c

@@ -3758,6 +3758,7 @@ register int osym, dmgtyp;
 	    obj2 = obj->nobj;
 	    if(obj->oclass != osym) continue; /* test only objs of type osym */
 	    if(obj->oartifact) continue; /* don't destroy artifacts */
+	    if(obj->in_use && obj->quan == 1) continue; /* not available */
 	    xresist = skip = 0;
 #ifdef GCC_WARN
 	    dmg = dindx = 0;
@@ -3829,6 +3830,7 @@ register int osym, dmgtyp;
 		    break;
 	    }
 	    if(!skip) {
+		if (obj->in_use) --quan; /* one will be used up elsewhere */
 		for(i = cnt = 0L; i < quan; i++)
 		    if(!rn2(3)) cnt++;