After flashing a bogus UBI image, e.g. when the UBI is bigger than the
available flash size, it can happen that barebox hangs itself with a
stacktrace when the board code or the environment does a ubi_attach() on
boot:

    ERROR: ubi0 error: init_volumes: not enough PEBs, required 7478, available 4024
    unable to handle NULL pointer dereference at address 0x00000204
    pc : [<1fd1eaf2>]    lr : [<1fd15aad>]
    sp : 1feefd30  ip : 0000000a  fp : 1fd5d1d7
    r10: 00001d34  r9 : 00000000  r8 : 1fd5d540
    r7 : 14983148  r6 : 00000006  r5 : ffffffe4  r4 : 14983004
    r3 : 1fd82bfc  r2 : 021f0000  r1 : 0000000a  r0 : 00000000
    Flags: nzcv  IRQs off  FIQs off  Mode SVC_32
    [<1fd1eaf2>] (ubi_fastmap_destroy_checkmap+0x8/0xa) from [<1fd15aad>] (ubi_read_volume_table+0x4d9/0x71c)
    [<1fd15aad>] (ubi_read_volume_table+0x4d9/0x71c) from [<1fd1cbf7>] (ubi_attach+0x157/0x1f8)
    [<1fd1cbf7>] (ubi_attach+0x157/0x1f8) from [<1fd16ec3>] (ubi_attach_mtd_dev+0x4a3/0x954)
    [...]

After this change, barebox at least boots to a prompt where the faulty
flash can be repaired:

    ERROR: ubi0 error: init_volumes: not enough PEBs, required 7478, available 4024
    ERROR: ubi0 error: ubi_attach_mtd_dev: failed to attach mtd0, error -28
    [...]
    running /env/bin/init...
    barebox@boardname:/

Signed-off-by: Roland Hieber <rhi@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>