tools/fit_image.c: Update some return code paths

Coverity has found some problems with the return paths in parts of this
code.  We have a case where we were going to the wrong part of the
unwind (open() failed so we cannot close the fd), a case where we were
only free()ing our buf on the error path and finally a case where we did
not munmap in the failure path.

Reported-by: Coverity (CID: 138492, 138495, 143064)
Signed-off-by: Tom Rini <trini@konsulko.com>