tools: kwbimage: Fix unchecked return value and fd leak

tools: kwbimage: Fix unchecked return value and fd leak

The return value of fstat was not checked in kwbimage, and in the case
of an error, the already open file was not closed. Fix both errors.

Reported-by: Coverity (CID: 155971)
Reported-by: Coverity (CID: 155969)
Signed-off-by: Mario Six <mario.six@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>