Fire trap on ice can indirectly kill a monster via drowning

Make the trap routine claim the trap killed the monster even
though it was drowning that did it, otherwise callers cannot
rely on the trap routine return value.

src/trap.c

@@ -1533,6 +1533,8 @@ trapeffect_fire_trap(
             You("smell smoke.");
         if (is_ice(tx, ty))
             melt_ice(tx, ty, (char *) 0);
+        if (DEADMONSTER(mtmp))
+            trapkilled = TRUE;
         if (see_it && t_at(tx, ty))
             seetrap(t_at(tx, ty));