Fix async callback handling issue

[medikoo]

Inner Logic of _executeQueryCommand result looks as:

var getMore = function(self, options, callback) { // ... self.db._executeQueryCommand(a, b, function(err, result) { // ... try { // ... callback(null, value); // ... } catch (e) { // ... callback(err); // ... } }); // ... };

It means that eventual error thrown within callback is propagated back to same callback. That's unexpected, additionally in that case async function callback is called twice, which should never happen.

This patch fixes that to:

self.db._executeQueryCommand(a, b, function(err, result) { var cbValue; // ... try { // ... cbValue = value; // ... } catch (e) { // ... callback(err); // ... } if (cbValue != null) callback(null, cbValue); });

Callback is not called with it's own error, and error is freely thrown as it should in such case.