fix panic in text formatter

The panic was caused to a nil pointer access when report
caller was activated with output coloring disabled

Fixes #852

logrus_test.go

@@ -702,3 +702,15 @@ func TestLogLevelEnabled(t *testing.T) {
 	assert.Equal(t, true, log.IsLevelEnabled(DebugLevel))
 	assert.Equal(t, true, log.IsLevelEnabled(TraceLevel))
 }
+
+func TestReportCallerOnTextFormatter(t *testing.T) {
+	l := New()
+
+	l.Formatter.(*TextFormatter).ForceColors = true
+	l.Formatter.(*TextFormatter).DisableColors = false
+	l.WithFields(Fields{"func": "func", "file": "file"}).Info("test")
+
+	l.Formatter.(*TextFormatter).ForceColors = false
+	l.Formatter.(*TextFormatter).DisableColors = true
+	l.WithFields(Fields{"func": "func", "file": "file"}).Info("test")
+}

text_formatter.go

@@ -164,18 +164,18 @@ func (f *TextFormatter) Format(entry *Entry) ([]byte, error) {
 	} else {
 		for _, key := range fixedKeys {
 			var value interface{}
-			switch key {
-			case f.FieldMap.resolve(FieldKeyTime):
+			switch {
+			case key == f.FieldMap.resolve(FieldKeyTime):
 				value = entry.Time.Format(timestampFormat)
-			case f.FieldMap.resolve(FieldKeyLevel):
+			case key == f.FieldMap.resolve(FieldKeyLevel):
 				value = entry.Level.String()
-			case f.FieldMap.resolve(FieldKeyMsg):
+			case key == f.FieldMap.resolve(FieldKeyMsg):
 				value = entry.Message
-			case f.FieldMap.resolve(FieldKeyLogrusError):
+			case key == f.FieldMap.resolve(FieldKeyLogrusError):
 				value = entry.err
-			case f.FieldMap.resolve(FieldKeyFunc):
+			case key == f.FieldMap.resolve(FieldKeyFunc) && entry.HasCaller():
 				value = entry.Caller.Function
-			case f.FieldMap.resolve(FieldKeyFile):
+			case key == f.FieldMap.resolve(FieldKeyFile) && entry.HasCaller():
 				value = fmt.Sprintf("%s:%d", entry.Caller.File, entry.Caller.Line)
 			default:
 				value = entry.Data[key]