[Hellfire bug] fix Fire Ring trap

MI_FireRing adds missiles MIS_FIREWALL with TARGET_BOTH and src equals
to either player ID, or -1 if Fire Ring is casted as trap. This has
consequences:
1.  Because TARGET_BOTH MonsterMHit was called, as it usually called for
Firewalls. During this calls reference to Player gets garbage data (id
-1). This may miscalculate:
    - no heal flags
    - knockback flags (already fixed in this PR)
    - unsquelch position.last
2. drops ear for firewall flame originating from Fire Ring from trap

Fix:
- Separated TARGET_BOTH from IsTrap for monsters to ensure TryHitMonster
will get passed TrapMissile (MonsterTrapHit) is called instead.
- added additional condition for earflag

Source/missiles.cpp

@@ -418,7 +418,7 @@ void CheckMissileCol(Missile &missile, int minDamage, int maxDamage, bool isDama
 				isPlayerHit = PlayerMHit(pid - 1, &monster, missile._midist, minDamage, maxDamage, missile._mitype, isDamageShifted, 0, &blocked);
 			}
 		} else {
-			int earflag = (missile._miAnimType == MFILE_FIREWAL || missile._miAnimType == MFILE_LGHNING) ? 1 : 0;
+			int earflag = (!missile.IsTrap() && (missile._miAnimType == MFILE_FIREWAL || missile._miAnimType == MFILE_LGHNING)) ? 1 : 0;
 			isPlayerHit = PlayerMHit(pid - 1, nullptr, missile._midist, minDamage, maxDamage, missile._mitype, isDamageShifted, earflag, &blocked);
 		}
 	}