Fix minor panic in http parser (#6750)

There was a bounds check error in parsing HTTP responses. A malformed
response line in the form "HTTP/1.1\r\n" would cause a panic when parsed.

Related to #6409

CHANGELOG.asciidoc

@@ -44,6 +44,8 @@ https://github.com/elastic/beats/compare/v5.6.8...5.6[Check the HEAD diff]
 - Fix corruption when parsing repeated headers in an HTTP request or response. {pull}6325[6325]
 - Fix panic when parsing partial AMQP messages. {pull}6384[6384]
 - Fix out of bounds access to slice in MongoDB parser. {pull}6256[6256]
+- Fix sniffer hanging on exit under Linux. {pull}6535[6535]
+- Fix bounds check error in http parser causing a panic. {pull}6750[6750]
 
 *Winlogbeat*
 

packetbeat/protos/http/http_parser.go

@@ -141,7 +141,7 @@ func (*parser) parseHTTPLine(s *stream, m *message) (cont, ok, complete bool) {
 	var version []byte
 	var err error
 	fline := s.data[s.parseOffset:i]
-	if len(fline) < 8 {
+	if len(fline) < 9 {
 		if isDebug {
 			debugf("First line too small")
 		}