[C] Use maxHealth instead of hard-coded value. Fixes BUKKIT-3998

#### The Issue:
This section of code uses the hardcoded maxHealth of 20, rather than the configurable maxHealth value.

#### Justification for this PR:
All other references to the hardcoded values have already been replaced by the getMaxHealth() calls

#### PR Breakdown:
This PR changes the call to use getMaxHealth() instead of a hard-coded value.

#### JIRA Ticket:
BUKKIT-3998 - https://bukkit.atlassian.net/browse/BUKKIT-3998