[openvswitch] catch cases when 'ovs-vsctl list bridge' has no output

[pmoravec]

If ovs_list_bridge_cmd does not return a valuable output, br_protos
variable won't be assigned before we reference it.

Fix that by assigning an empty list to br_protos .

Resolves: #2249

Signed-off-by: Pavel Moravec pmoravec@redhat.com

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