API: Detect wrong JSON and return correct error message

application/controllers/Api.php

@@ -421,7 +421,10 @@ class API extends CI_Controller {
 
 		// Decode JSON and store
 		$obj = json_decode(file_get_contents("php://input"), true);
-
+		if ($obj === NULL) {
+		    echo json_encode(['status' => 'failed', 'reason' => "wrong JSON"]);
+		    die();
+		}
 
 		if(!isset($obj['key']) || $this->api_model->authorize($obj['key']) == 0) {
 		   http_response_code(401);