[C] Fixed negative damage (Zombie) - Fixes BUKKIT-4193

**The Issue:**
Currently the method used for calculating the zombies damage is health scaled, but it uses the default max health value instead of the real max health value.
If the zombies have more than default max health (20), the calculated damage become negative.

(maxHealth - health) / maxHealth
(20 - 100) / 20 = -4
Proper handling would use the real maxHealth value.
(100 - 100) /100 = 0

I also checked all other entities, the zombie is the only one with this kind of issue.

Some debug output i used to find the bug source: 
Class.method() = relevant damage value (every change listed) 
Executed with custom maxHealth (100/100)
[INFO] EntityZombie.c() = -4.0
[INFO] EntityZombie.c() = -13
[INFO] EntityZombie.c() = -6
[INFO] EntityMonster.m() = -6
[INFO] EntityPlayer.damageEntity() = -6
[INFO] EntityHuman.damageEntity() = -6
[INFO] EntityHuman.damageEntity() = -2
[INFO] callDamageEvent

Executed with default health  (19/20)
[INFO] EntityZombie.c() = 0.05
[INFO] EntityZombie.c() = 3
[INFO] EntityZombie.c() = 3
[INFO] EntityMonster.m() = 3
[INFO] EntityPlayer.damageEntity() = 3
[INFO] EntityHuman.damageEntity() = 3
[INFO] EntityHuman.damageEntity() = 2
[INFO] callDamageEvent

**First affected version/commit:**
Update CraftBukkit to Minecraft 1.5
https://github.com/Bukkit/CraftBukkit/commit/d3d98a166f05f8fadfcd11adc0318c548da8a25b#L137R140

**Justification for this PR:**
This is a bug caused by adding maxHealth feature in Bukkit.

**PR Breakdown:**
Fixed negative damage.

JIRA: https://bukkit.atlassian.net/browse/BUKKIT-4193