video: fbdev: fbmem: fix pointer reference to null device field

In function do_remove_conflicting_framebuffers, if device is NULL, there
will be null pointer reference. The patch add a check to the if expression.

Signed-off-by: Zhouyi Zhou <zhouzhouyi@gmail.com>
Acked-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Helge Deller <deller@gmx.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20220210065824.368355-1-zhouzhouyi@gmail.com