Fix bz65: Water behaves strangely when hurtling out of air bubble

When levitating on the plane of water, get an unattached iron ball,
and throw it. You'll hurtle in the opposite direction, out of the air
bubble and into water, but don't drown.

src/dothrow.c

@@ -621,6 +621,12 @@ int x, y;
     newsym(ox, oy);    /* update old position */
     vision_recalc(1);  /* update for new position */
     flush_screen(1);
+
+    if (levl[x][y].typ == WATER && Is_waterlevel(&u.uz)) {
+        drown();
+        return FALSE;
+    }
+
     /* FIXME:
      * Each trap should really trigger on the recoil if
      * it would trigger during normal movement. However,