don't call the callback twice in case it fails

lib/mbtiles.js

@@ -253,10 +253,11 @@ MBTiles.prototype.grid = function(x, y, z, callback) {
                         memo[r.key_name] = JSON.parse(r.key_json);
                         return memo;
                     }, {});
-                callback(null, _(JSON.parse(grid)).extend({ data: data }));
             } catch (err) {
-                callback('Grid is invalid');
+                return callback('Grid is invalid');
             }
+
+            callback(null, _(JSON.parse(grid)).extend({ data: data }));
         }
     );
 };