Fix 'index out of range' bug in Packetbeat DNS protocol plugin

When converting resource records into MapStrs the code can run into an index out of range exception when processing OPT psuedo resource records. The code reduces the size of a slice by creating a new, smaller slice and copies the existing data to the smaller slice. But the code did not account for the fact that the length of the slice was smaller when indexing.

Fixes elastic#2872

(cherry picked from commit cbde106)

CHANGELOG.asciidoc

@@ -35,6 +35,8 @@ https://github.com/elastic/beats/compare/v5.0.0...5.0[Check the HEAD diff]
 
 *Packetbeat*
 
+- Fix 'index out of bounds' bug in Packetbeat DNS protocol plugin. {issue}2872[2872]
+
 *Topbeat*
 
 *Filebeat*

packetbeat/protos/dns/dns.go

@@ -529,26 +529,23 @@ func optToMapStr(rrOPT *mkdns.OPT) common.MapStr {
 	return optMapStr
 }
 
-// rrsToMapStr converts an array of RR's to an array of MapStr's.
+// rrsToMapStr converts an slice of RR's to an slice of MapStr's.
 func rrsToMapStrs(records []mkdns.RR) []common.MapStr {
-	mapStrArray := make([]common.MapStr, len(records))
-	for i, rr := range records {
+	mapStrSlice := make([]common.MapStr, 0, len(records))
+	for _, rr := range records {
 		rrHeader := rr.Header()
 
 		mapStr := rrToMapStr(rr)
 		if len(mapStr) == 0 { // OPT pseudo-RR returns an empty MapStr
-			resizeStrArray := make([]common.MapStr, len(mapStrArray)-1)
-			copy(resizeStrArray, mapStrArray)
-			mapStrArray = resizeStrArray
 			continue
 		}
 		mapStr["name"] = rrHeader.Name
 		mapStr["type"] = dnsTypeToString(rrHeader.Rrtype)
 		mapStr["class"] = dnsClassToString(rrHeader.Class)
 		mapStr["ttl"] = strconv.FormatInt(int64(rrHeader.Ttl), 10)
-		mapStrArray[i] = mapStr
+		mapStrSlice = append(mapStrSlice, mapStr)
 	}
-	return mapStrArray
+	return mapStrSlice
 }
 
 // Convert all RDATA fields of a RR to a single string