fix https://github.com/sagemathinc/smc/issues/1756 -- "syncdoc/get_users_cursors: Cannot read property 'get_cursors' of undefined"
fix https://github.com/sagemathinc/smc/issues/1756 -- "syncdoc/get_users_cursors: Cannot read property 'get_cursors' of undefined"

sagemathinc/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,DrXyzzy/smc,sagemathinc/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,sagemathinc/smc,tscholl2/smc,DrXyzzy/smc