[U-Boot] [U-Boot, v4] efi_loader: fix off-by-one bug in efi_get_variable

[Alexander Graf]

> efi_get_variable() always stores an extra zero byte after the output data.
> When the returned data size matches the output buffer size, the extra zero
> byte is stored past the end of the output buffer.
> 
> Signed-off-by: Ivan Gorinov <ivan.gorinov at intel.com>
> Reviewed-by: Heinrich Schuchardt <xypron.glpk at gmx.de>

Thanks, applied to efi-next

Alex