idirectfbdisplaylayer: check if data->stack isn't NULL before calling any cursor routines
idirectfbdisplaylayer: check if data->stack isn't NULL before calling any cursor routines

For example, calling IDirectFBDisplayLayer_EnableCursor() leads to a SIGSEGV within
dfb_windowstack_lock() because of a deference of the uninitialized CoreWindowStack *stack
variable.

The patch introduces few more additional checks, just like what has been implemented in
IDirectFBDisplayLayer_SetBackgroundMode() and IDirectFBDisplayLayer_SetBackgroundImage().

Signed-off-by: Ilyes Gouta <7234d5fbd372cd5418108eb00f265b68ffb8d93b@st.com>

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