veyron/services/store/memstore/query: Fix race condition.

Change-Id: I2168f89fece19c50937467200bd0cdf34031e3e6
diff --git a/services/store/memstore/query/eval.go b/services/store/memstore/query/eval.go
index 7061a67..366b348 100644
--- a/services/store/memstore/query/eval.go
+++ b/services/store/memstore/query/eval.go
@@ -80,9 +80,13 @@
 
 // Next implements the Iterator method.
 func (it *evalIterator) Next() bool {
+	it.mu.Lock()
 	if it.err != nil {
+		it.mu.Unlock()
 		return false
 	}
+	it.mu.Unlock()
+
 	select {
 	case result, ok := <-it.results:
 		if !ok {