Consecutive indirect branches are not handled properly

[ayrtonm]

When the plugin encounters consecutive indirect branches the the indirect_branch_exec callback is registered for both, but branch_skipped is also registered for the second. This means that results may vary depending on which callback is executed first.

This could be fixed by not registering the branch_skipped for the second instruction (i.e. the branch_skipped callback that corresponds to skipping the first branch). Since this scenario is rare in practice, the plugin currently just emits a warning to stdout when it runs into this. It'd be good to have some test cases before making the fix to verify it'll work as expected.