fix another part of #2629 - destory_mitem of partial stack (trunk only)

From a bug report, if part of a stack in a monster's inventory gets
destroyed (examples given were from Fire Brand burning scrolls or Frost
Brand freezing potions, but other code calls destroy_mitem() too), the
message handled singular vs plural for the desruction part ("catches
fire and burns" vs "catch fire and burn") but always used plural when
formatting the object stack being affected.  Now say "One of <the mon's
foo>" or "Some of <the mon's foo>" as appropriate when part of a stack
is being destroyed.  (destroy_item() for the hero paid more attention
and handled this issue correctly.)

doc/fixes35.0

@@ -412,6 +412,8 @@ theft of worn armor with wear/unwear delay would interfere with completion of
 alchemical explosion or evaporation only used up one potion instead of all
 	the potions being dipped
 feedback for reverse-genocide was plural even when just one monster was created
+fix message given when part of a stack of items in a monster's inventory is
+	being destroyed
 
 
 Platform- and/or Interface-Specific Fixes

src/zap.c

@@ -4587,7 +4587,10 @@ int osym, dmgtyp;
 		    if(!rn2(3)) cnt++;
 
 		if(!cnt) continue;
-		if (vis) pline("%s %s!", Yname2(obj),
+		if (vis) pline("%s%s %s!",
+			       (cnt == obj->quan) ? "" :
+				 (cnt > 1L) ? "Some of " : "One of ",
+			       (cnt == obj->quan) ? Yname2(obj) : yname(obj),
 			       destroy_strings[dindx][(cnt > 1L)]);
 		for(i = 0; i < cnt; i++) m_useup(mtmp, obj);
 	    }