Fixed a missing block bug

Description: w.mu is locked at line 385 and unlocked at line 396. Among 5 return statements in this function, 4 are below line 396 but there is 1 return at line 387. 
Fix: Add w.mu.Unlock() before that return at line 387.

clientv3/watch.go

@@ -384,6 +384,7 @@ func (w *watcher) RequestProgress(ctx context.Context) (err error) {
 
 	w.mu.Lock()
 	if w.streams == nil {
+		w.mu.Unlock()
 		return fmt.Errorf("no stream found for context")
 	}
 	wgs := w.streams[ctxKey]