fix #K3968 - objfree: obj not worn

New feature to sometimes hit twice for skilled martial-arts/bare-handed
was unconditionally using uswapwep for the second hit.  If it was a
breakable object, hitting could break it and produce impossible "objfree:
obj not free".

Only use uswapwep for u.twoweap; use Null for second bare-handed hit.

doc/fixes3-7-0.txt

@@ -1649,6 +1649,9 @@ hero might not be credited with "entered Mine Town" achievement for the town
 in the theme room "water-surrounded vault", guarantee an item which can be
 	be used to escape from the room in one of the four chests
 pets capable of using items would pick up and wear cursed armor
+if something breakable was set up as alternate weapon and the second of two
+	bare-handed hits succeeded, it would be broken and trigger impossible
+	"objfree: deleting worn obj"
 
 
 Fixes to 3.7.0-x Platform and/or Interface Problems Exposed Via git Repository

src/uhitm.c

@@ -743,7 +743,8 @@ static boolean
 hitum(struct monst *mon, struct attack *uattk)
 {
     boolean malive, wep_was_destroyed = FALSE;
-    struct obj *wepbefore = uwep;
+    struct obj *wepbefore = uwep,
+        *secondwep = u.twoweap ? uswapwep : (struct obj *) 0;
     int tmp, dieroll, mhit, armorpenalty, attknum = 0,
         x = u.ux + u.dx, y = u.uy + u.dy, oldumort = u.umortality;
 
@@ -787,11 +788,12 @@ hitum(struct monst *mon, struct attack *uattk)
         mon_maybe_unparalyze(mon);
         dieroll = rnd(20);
         mhit = (tmp > dieroll || u.uswallow);
-        malive = known_hitum(mon, uswapwep, &mhit, tmp, armorpenalty, uattk,
+        malive = known_hitum(mon, secondwep, &mhit, tmp, armorpenalty, uattk,
                              dieroll);
         /* second passive counter-attack only occurs if second attack hits */
         if (mhit)
-            (void) passive(mon, uswapwep, mhit, malive, AT_WEAP, !uswapwep);
+            (void) passive(mon, secondwep, mhit, malive, AT_WEAP,
+                           secondwep && !uswapwep);
     }
     gt.twohits = 0;
     return malive;