Set session[:snap_selected] to nil if Snapshot does not exist

https://bugzilla.redhat.com/show_bug.cgi?id=1395116

app/controllers/vm_common.rb

@@ -269,6 +269,7 @@ def show(id = nil)
     elsif @display == "snapshot_info"
       drop_breadcrumb(:name => @record.name + _(" (Snapshots)"),
                       :url  => "/#{rec_cls}/show/#{@record.id}?display=#{@display}")
+      session[:snap_selected] = nil if Snapshot.find_by(:id => session[:snap_selected]).nil?
       @sb[@sb[:active_accord]] = TreeBuilder.build_node_id(@record)
       @snapshot_tree = TreeBuilderSnapshots.new(:snapshot_tree, :snapshot, @sb, true, :root => @record)
       @active = if @snapshot_tree.selected_node