don't call the callback twice in case it fails

Conflicts:

	lib/mbtiles.js

lib/mbtiles.js

@@ -342,10 +342,11 @@ MBTiles.prototype.getGrid = function(z, x, y, callback) {
                         memo[r.key_name] = JSON.parse(r.key_json);
                         return memo;
                     }, {});
-                callback(null, _(JSON.parse(grid)).extend({ data: data }));
             } catch (err) {
-                callback(new Error('Grid is invalid'));
+                return callback(new Error('Grid is invalid'));
             }
+
+            callback(null, _(JSON.parse(grid)).extend({ data: data }));
         }
     );
 };