futility: fix segfault on short files

Verify the size of the buffer read from the file before trying to use 1KB of
it for the new rwsig format detection.

Add a new test case with a short file containing only 4 bytes of unknown
data and run "futility show" on it.

BRANCH=smaug
BUG=none
TEST=futility show foobar.pub.pem
where foobar.pub.pem is a 451-byte file.
check that "make runtests" passes with the fix
and fails without it with the following message :
test_file_types.sh ... failed
FAIL: 13 / 14 passed

Change-Id: Ia9d68c6b528c2b3a595ea6791c907374616d051f
Reviewed-on: https://chromium-review.googlesource.com/306682
Commit-Ready: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Bill Richardson <wfrichar@chromium.org>
3 files changed