Painting: fix dropping multiple items when multiple blockupdates occur in the same tick

test case: place sand on top of a line of signs, put a painting on the sand and break the supporting sign
fixes #2774 for stable

src/pocketmine/entity/object/Painting.php

@@ -90,6 +90,9 @@ class Painting extends Entity{
 	}
 
 	public function kill() : void{
+		if(!$this->isAlive()){
+			return;
+		}
 		parent::kill();
 
 		$drops = true;