[U-Boot] efi_loader: Exit in efi_set_bootdev() upon invalid "desc"

[Alexander Graf]

> When trying to load an image from a non-existent USB key, U-Boot v2017.11
> crashes on my x86 platform:
> 
> => load usb 0:1 03000000 abc
> General Protection
> EIP: 0010:[<7b59030d>] EFLAGS: 00010286
> Original EIP :[<fff4330d>]
> ...
> 
> This used to work in v2017.09. Testing has shown, that this bug was
> introduced with patch 95c5553e [efi_loader: refactor boot device and
> loaded_image handling].
> 
> This patch now checks if a valid "desc" is returned from blk_get_dev()
> and only continues when "desc" is available. Resulting in this cmd
> output (again):
> 
> => load usb 0:1 03000000 abc
> ** Bad device usb 0 **
> 
> Signed-off-by: Stefan Roese <sr at denx.de>
> Cc: Rob Clark <robdclark at gmail.com>
> Cc: Heinrich Schuchardt <xypron.glpk at gmx.de>
> Cc: Alexander Graf <agraf at suse.de>
> Cc: Marek Vasut <marex at denx.de>
> Cc: Bin Meng <bmeng.cn at gmail.com>
> Reviewed-by: Heinrich Schuchardt <xypron.glpk at gmx.de>

Thanks, applied to efi-next

Alex