UBI: Fix crash in try_recover_peb()

    drivers/mtd/ubi/eba.c: In function ‘try_recover_peb’:
    drivers/mtd/ubi/eba.c:744: warning: ‘vid_hdr’ is used uninitialized in this function

The pointer vid_hdr is indeed not initialized, leading to a crash when
it is dereferenced.

Fix this by obtaining the pointer from the VID buffer, like is done
everywhere else.

Fixes: 3291b52f

 ("UBI: introduce the VID buffer concept")
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Richard Weinberger <richard@nod.at>