handle nil pointer in ec2 discovery (#4469)

This handles a nil pointer that was being accessed in EC2 discovery.

Fixes: #4441

Signed-off-by: noqcks <benny@noqcks.io>

discovery/ec2/ec2.go

@@ -263,6 +263,9 @@ func (d *Discovery) refresh() (tg *targetgroup.Group, err error) {
 					var subnets []string
 					subnetsMap := make(map[string]struct{})
 					for _, eni := range inst.NetworkInterfaces {
+						if eni.SubnetId == nil {
+							continue
+						}
 						if _, ok := subnetsMap[*eni.SubnetId]; !ok {
 							subnetsMap[*eni.SubnetId] = struct{}{}
 							subnets = append(subnets, *eni.SubnetId)