[C] Correct casting of coordinates when dismounting. Fixes BUKKIT-5288

###### The Issue:
When exiting a minecart/boat/horse, the player may be left on an unsafe place.

When X or Z are negative, the game is off by 1 when checking possible exit places.

###### PR Breakdown:
This misbehaviour is due to lack of coordinates floor()'ing when casting to integer.
The '<' becoming '<=' seems obvious and enable 3 more possible destinations.

###### JIRA Ticket:
BUKKIT-5288 https://bukkit.atlassian.net/browse/BUKKIT-5288