avoid crash/exception with the getTile() method by adding some map size check
avoid crash/exception with the getTile() method by adding some map size check

note : this is not used internally by the TMX renderer, so it won't
impact performances.

melonjs/melonJS,rlugojr/melonJS,rlugojr/melonJS,rlugojr/melonJS,melonjs/melonJS