core: prevent segfault when calls are launched from non-dfb threads
core: prevent segfault when calls are launched from non-dfb threads

kevleyski/directfb,jcdubois/DirectFB,mtsekm/test,sklnet/DirectFB,sklnet/DirectFB,kevleyski/directfb,djbclark/directfb-core-DirectFB,kevleyski/DirectFB-1,Distrotech/DirectFB,sklnet/DirectFB,djbclark/directfb-core-DirectFB,deniskropp/DirectFB,dfbdok/DirectFB1,kaostao/directfb,kevleyski/DirectFB-1,lancebaiyouview/DirectFB,DirectFB/directfb,DirectFB/directfb,djbclark/directfb-core-DirectFB,kevleyski/DirectFB-1,jcdubois/DirectFB,kevleyski/directfb,sklnet/DirectFB,kaostao/directfb,mtsekm/test,DirectFB/directfb,lancebaiyouview/DirectFB,kevleyski/directfb,mtsekm/test,Distrotech/DirectFB,lancebaiyouview/DirectFB,Distrotech/DirectFB,kevleyski/DirectFB-1,kaostao/directfb,djbclark/directfb-core-DirectFB,deniskropp/DirectFB,jcdubois/DirectFB,lancebaiyouview/DirectFB,deniskropp/DirectFB,dfbdok/DirectFB1,dfbdok/DirectFB1,deniskropp/DirectFB