Make air elementals respect damage reduction from AC

Players of both 3.4.3 and 3.7 have observed for a long time that
air elementals are disproportionately dangerous compared to other
endgame threats. (In particular, playtester feedback from 3.7 was
that the Plane of Air was much more dangerous than it should be,
with playtesters treating it similarly to Astral with respect to
the use of high-quality escape items.)

It turns out that this was because damage from air elementals while
engulfed was entirely ignoring AC, meaning that regardless of your
stats, you would be taking around 16.5 damage per turn while
engulfed (half physical damage helped, but nothing else did).

This commit purely fixes the bug, but does not balance around it,
which means that it causes the endgame air elementals to become
almost entirely nonthreatening. In a future commit, I plan to
balance around this change.

src/mhitu.c

@@ -1535,8 +1535,15 @@ gulpmu(struct monst *mtmp, struct attack *mattk)
         break;
     }
 
-    if (physical_damage)
+    if (physical_damage) {
+        /* same damage reduction for AC as in hitmu */
+        if (u.uac < 0)
+            tmp -= rnd(-u.uac);
+        if (tmp < 0)
+            tmp = 1;
+
         tmp = Maybe_Half_Phys(tmp);
+    }
 
     gm.mswallower = mtmp; /* match gulpmm() */
     mdamageu(mtmp, tmp);