kwbimage: Fix out of bounds access

The kwbimage format is reading beyond its header structure if it
misdetects a Xilinx Zynq image and tries to read it. Fix it by
sanity checking that the header we want to read fits inside our
file size.

Signed-off-by: Alexander Graf <agraf@suse.de>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>