Fix dmonsfree warning caused by knockback

When a monster being attacked was knocked back into a level
teleport trap, the attacker could still hit the defender.
If the second hit then killed the defender, this could result
in dmonsfree warning.

include/monst.h

@@ -241,6 +241,8 @@ struct monst {
 #define engulfing_u(mon) (u.uswallow && (u.ustuck == (mon)))
 #define helpless(mon) ((mon)->msleeping || !(mon)->mcanmove)
 
+#define mon_offmap(mon) (((mon)->mstate & (MON_DETACH|MON_MIGRATING|MON_LIMBO|MON_OFFMAP)) != 0)
+
 /* Get the maximum difficulty monsters that can currently be generated,
    given the current level difficulty and the hero's level. */
 #define monmax_difficulty(levdif) (((levdif) + u.ulevel) / 2)

src/mhitm.c

@@ -559,6 +559,9 @@ mattackm(register struct monst *magr, register struct monst *mdef)
         /* return if aggressor can no longer attack */
         if (helpless(magr))
             return res[i];
+        /* eg. defender was knocked into a level teleport trap */
+        if (mon_offmap(mdef))
+            return res[i];
         if (res[i] & M_ATTK_HIT)
             struck = 1; /* at least one hit */
     } /* for (;i < NATTK;) loop */

src/mon.c

@@ -989,7 +989,7 @@ movemon_singlemon(struct monst *mtmp)
         return FALSE;
 
     /* monster isn't on this map anymore */
-    if ((mtmp->mstate & (MON_DETACH|MON_MIGRATING|MON_LIMBO|MON_OFFMAP)) != 0)
+    if (mon_offmap(mtmp))
         return FALSE;
 
     /* Find a monster that we have not treated yet. */