Don't attempt to cache encumber_msg result

There was only one point in the code at which this caching was
being done, and it was incorrect: it's possible for the result of
near_capacity to change during a monster turn because monster
actions can change either inventory weight or carry capacity.

The bug was particularly relevant in cases where a character
polymorphed into a slow weak monster gets attacked by a monster
that moves at normal speed: due to the polyform being slow, the
normal-speed monster gets in a lot of attacks and causes a
rehumanization, but due to the polyform being weak, it was
burdened at the start of the monster turn, and so when that
penalty is (due to the bug) applied to the next turn it can
mean that the character misses the next turn too, and may end up
dying as a result.

src/mkobj.c

@@ -1657,7 +1657,7 @@ shrink_glob(
     }
     if (updinv) {
         update_inventory();
-        (void) encumber_msg();
+        encumber_msg();
     }
 }
 
@@ -2892,7 +2892,7 @@ hornoplenty(
             /* item still in magic horn was weightless; when it's now in
                a carried container, hero's encumbrance could change */
             if (carried(targetbox)) {
-                (void) encumber_msg();
+                encumber_msg();
                 update_inventory(); /* for contents count or wizweight */
             }
         } else {