tools: dumpimage: Fix crashing when trying to extract data from kwbimage

tools: dumpimage: Fix crashing when trying to extract data from kwbimage

Trying to call the following command causes NULL pointer dereference in
strlen():
  ./tools/dumpimage -T kwbimage -o /tmp/out u-boot-spl.kwb

Fix it by checking whether params->imagename is non-NULL before calling
strlen().

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>