gfxmenu/gui_image: Fix double free of bitmap

gfxmenu/gui_image: Fix double free of bitmap

In grub-core/gfxmenu/gui_image.c, Coverity detected a double free in the
function load_image(). The function checks if self->bitmap and self->raw_bitmap
aren't NULL and then frees them. In the case self->bitmap and self->raw_bitmap
are the same, only self->raw_bitmap is freed which would also free the memory
used by self->bitmap. However, in this case self->bitmap isn't being set to NULL
which could lead to a double free later in the code. After self->raw_bitmap is
freed, it gets set to the variable bitmap. If this variable is NULL, the code
could have a path that would free self->bitmap a second time in the function
rescale_image().

Fixes: CID 292472
Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>