fix race condition in entry

entry.go

@@ -219,6 +219,8 @@ func (entry Entry) HasCaller() (has bool) {
 // This function is not declared with a pointer value because otherwise
 // race conditions will occur when using multiple goroutines
 func (entry Entry) log(level Level, msg string) {
+	entry.Logger.mu.Lock()
+	defer entry.Logger.mu.Unlock()
 	var buffer *bytes.Buffer
 
 	// Default to now, but allow users to override if they want.