fix: lock buffer during write before read

The formatters write to the internal buffer. We need to guard that
before we read the buffer.

Fixes: #67

logger.go

@@ -58,8 +58,6 @@ func (l *Logger) log(level Level, msg interface{}, keyvals ...interface{}) {
 		return
 	}
 
-	l.mu.Lock()
-	defer l.mu.Unlock()
 	defer l.b.Reset()
 
 	var kvs []interface{}
@@ -98,6 +96,7 @@ func (l *Logger) log(level Level, msg interface{}, keyvals ...interface{}) {
 		kvs = append(kvs, ErrMissingValue)
 	}
 
+	l.mu.Lock()
 	switch l.formatter {
 	case LogfmtFormatter:
 		l.logfmtFormatter(kvs...)
@@ -106,6 +105,7 @@ func (l *Logger) log(level Level, msg interface{}, keyvals ...interface{}) {
 	default:
 		l.textFormatter(kvs...)
 	}
+	l.mu.Unlock()
 
 	_, _ = l.w.Write(l.b.Bytes())
 }