Fuzzing test for syscall exec

[mohanson]

What problem does this PR solve?

Problem Summary:

Add fuzz test for exec syscall and fix the problems found in the test. Test results: https://github.com/mohanson/ckb_vm_exec_fuzz/tree/master/v1

What is changed and how it works?

What's Changed:

• exec always return err code and Ok(true)
• add bound check for exec's offset and length

Related changes

• PR to update owner/repo:
• Need to cherry-pick to the release branch

Check List

Tests

• Unit test

Release note

None

[quake]

LGTM, btw, is the out-of-bound issue catched by fuzz test?

[mohanson]

LGTM, btw, is the out-of-bound issue catched by fuzz test?

By my new unit test check_exec_big_offset_length

[driftluo]

bors r=quake,driftluo

[bors]

Build succeeded:

• ci