cmd: fix panic on empty peer string
cmd: fix panic on empty peer string

brunoqc/alertmanager,prometheus/alertmanager,joehandzik/alertmanager,joehandzik/alertmanager,joehandzik/alertmanager,Aloomaio/alertmanager,Aloomaio/alertmanager,Kellel/alertmanager,prometheus/alertmanager,tinytub/alertmanager,GoogleCloudPlatform/alertmanager,tyrken/alertmanager,Kellel/alertmanager,GoogleCloudPlatform/alertmanager,prometheus/alertmanager,tyrken/alertmanager,tinytub/alertmanager,Aloomaio/alertmanager,brunoqc/alertmanager,brunoqc/alertmanager,tyrken/alertmanager,tinytub/alertmanager,prometheus/alertmanager,GoogleCloudPlatform/alertmanager,GoogleCloudPlatform/alertmanager,Kellel/alertmanager,tyrken/alertmanager,brunoqc/alertmanager