Fix JSON panic (elastic#4042)

There can be a panic in the JSON decoding code if the input JSON line contains
"null" as a string, because that unmarshals without errors but results in a nil
map.

CHANGELOG.asciidoc

@@ -60,6 +60,7 @@ https://github.com/elastic/beats/compare/v5.1.1...master[Check the HEAD diff]
 - Allow `-` in Apache access log byte count. {pull}3863[3863]
 - Downgrade Elasticsearch per batch item failure log to debug level. {issue}3953[3953]
 - Allow log lines without a program name in the Syslog fileset. {pull}3944[3944]
+- Fix panic in JSON decoding code if the input line is "null". {pull}4042[4042]
 
 *Heartbeat*
 - Add default ports in HTTP monitor. {pull}3924[3924]

filebeat/harvester/reader/json.go

@@ -30,7 +30,7 @@ func (r *JSON) decodeJSON(text []byte) ([]byte, common.MapStr) {
 	var jsonFields map[string]interface{}
 
 	err := unmarshal(text, &jsonFields)
-	if err != nil {
+	if err != nil || jsonFields == nil {
 		logp.Err("Error decoding JSON: %v", err)
 		if r.cfg.AddErrorKey {
 			jsonFields = common.MapStr{JsonErrorKey: fmt.Sprintf("Error decoding JSON: %v", err)}

filebeat/harvester/reader/json_test.go

@@ -116,6 +116,13 @@ func TestDecodeJSON(t *testing.T) {
 			ExpectedText: `{"message": "test", "value": "`,
 			ExpectedMap:  nil,
 		},
+		{
+			// in case the JSON is "null", we should just not panic
+			Text:         `null`,
+			Config:       JSONConfig{MessageKey: "value", AddErrorKey: true},
+			ExpectedText: `null`,
+			ExpectedMap:  common.MapStr{"json_error": "Error decoding JSON: <nil>"},
+		},
 		{
 			// Add key error helps debugging this
 			Text:         `{"message": "test", "value": "`,