tools, rsa: Further minor cleanups on top of c236ebd2 and 2b9ec7

[NOTE: I took v1 of these patches in, and then v2 came out, this commit
is squashing the minor deltas from v1 -> v2 of updates to c236ebd2 and
2b9ec762 into this commit - trini]

- Added an additional NULL check, as suggested by Simon Glass to
  fit_image_process_sig
- Re-formatted the comment blocks

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>
[For merging the chnages from v2 back onto v1]
Signed-off-by: Tom Rini <trini@konsulko.com>