Fix Medi-kit exploit.

In CE, you cannot exceed maximum health.
In early DOS versions, it was sometimes possible to overflow the health stat so the soldier would be healthier after healing than he started before he was shot.
http://ufopaedia.org/index.php?title=Medi-Kit_(EU)
Merge pull request #344 from redv/medikit_patch

Fix Medi-Kit exploit.