kwbimage: check fopen() return value

Always check the return value of fopen().

This resolves Coverity CID 338491:
Null pointer dereferences (NULL_RETURNS)

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