[crash] Move control-offset check into loop
[crash] Move control-offset check into loop

We checked this once outside the loop, but we need to check it on every
iteration in case the interior control offsets are corrupt.

@Manishearth: Here's one for the `cargo-fuzz` trophy case!  It's working
great now.  I'll submit a PR with a list of everything I find.

emk/substudy,emk/substudy,emk/substudy,emk/substudy,emk/substudy