MokManager: check_der_suffix(): use StrnCat() on our suffix checker.
MokManager: check_der_suffix(): use StrnCat() on our suffix checker.

We know it's legit already because we computed the pointer from the end,
but covscan gets confused, and we have StrnCat, so we should just use it
anyway.

Signed-off-by: Peter Jones <2e0a021e603479ff81838c00c3b770daca4e0214@redhat.com>

rhinstaller/shim,rhinstaller/shim,rhinstaller/shim