kwbimage: check return value of image_get_csk_index

image_get_csk_index() may return -1 in case of an error. Don't use this
value as index.

This resolves Coverity CID 338488
Memory - illegal accesses  (NEGATIVE_RETURNS)

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Pali Rohár <pali@kernel.org>