Fix for panic when ssh min and max port is the same for qemu builder. Taken...

Fix for panic when ssh min and max port is the same for qemu builder. Taken directly from a previous patch by mitchellh 8f50d2dd9a5b943881f0827fc07fbfb389e1407c

builder/qemu/step_forward_ssh.go

@@ -2,11 +2,12 @@ package qemu
 
 import (
 	"fmt"
-	"github.com/mitchellh/multistep"
-	"github.com/mitchellh/packer/packer"
 	"log"
 	"math/rand"
 	"net"
+
+	"github.com/mitchellh/multistep"
+	"github.com/mitchellh/packer/packer"
 )
 
 // This step adds a NAT port forwarding definition so that SSH is available
@@ -23,9 +24,16 @@ func (s *stepForwardSSH) Run(state multistep.StateBag) multistep.StepAction {
 
 	log.Printf("Looking for available SSH port between %d and %d", config.SSHHostPortMin, config.SSHHostPortMax)
 	var sshHostPort uint
+	var offset uint = 0
+
 	portRange := int(config.SSHHostPortMax - config.SSHHostPortMin)
+	if portRange > 0 {
+		// Have to check if > 0 to avoid a panic
+		offset = uint(rand.Intn(portRange))
+	}
+
 	for {
-		sshHostPort = uint(rand.Intn(portRange)) + config.SSHHostPortMin
+		sshHostPort = offset + config.SSHHostPortMin
 		log.Printf("Trying port: %d", sshHostPort)
 		l, err := net.Listen("tcp", fmt.Sprintf(":%d", sshHostPort))
 		if err == nil {