u.uswallow

Make sure u.uswallow is cleared when u.ustuck gets set to Null so
that they won't be out of sync with each other.  Having u.uswallow
be non-zero does imply that u.ustuck is non-Null.

Running #panic while swallowed didn't produce any anomalies for me,
either before or after this change.

src/save.c

@@ -183,7 +183,7 @@ dosave0(void)
     /* these pointers are no longer valid, and at least u.usteed
      * may mislead place_monster() on other levels
      */
-    set_ustuck((struct monst *) 0);
+    set_ustuck((struct monst *) 0); /* also clears u.uswallow */
     u.usteed = (struct monst *) 0;
 
     for (ltmp = (xint8) 1; ltmp <= maxledgerno(); ltmp++) {