core: fix dfb_core_destroy() logic which failed if there were multiple references on the core
core: fix dfb_core_destroy() logic which failed if there were multiple references on the core

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