tools: kwbimage: Don't crash when binary file name does not contain '/'

In the case when the file name is specified relative to the current
working directory, it does not contain '/' character and strrchr()
returns NULL.

The following strcmp() function then crashes on NULL pointer
dereference.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>