RegionLocationTableEntry: cap firstSector at 16777216

this is the biggest sector start that the location table can represent, due to the binary format. Larger values than this will overflow and cause corruption.
This provides an effective limit of 64 GB on region files.

src/pocketmine/level/format/io/region/RegionLocationTableEntry.php

@@ -38,7 +38,7 @@ class RegionLocationTableEntry{
 	 * @throws \InvalidArgumentException
 	 */
 	public function __construct(int $firstSector, int $sectorCount, int $timestamp){
-		if($firstSector < 0){
+		if($firstSector < 0 or $firstSector >= 2 ** 24){
 			throw new \InvalidArgumentException("Start sector must be positive, got $firstSector");
 		}
 		$this->firstSector = $firstSector;