data/bypassing-stack-protector: Fix canary overwrite

[XXMA16]

Prerequisite Checklist

• Read the contribution guidelines regarding submitting new changes to the project;
• Tested your changes against relevant architectures and platforms;
• Updated relevant documentation (if needed).

Description of changes

The canary and return address overwrites were done two frames higher (instead of only one).

An explicit statement that the canary overwrite should be commented out before proceeding with replacing the return address with &pawned has also been added.

Explanation

addr points to [RBP-16], since at [RBP-8] resides the canary, meaning that addr[1] will overwrite it. The return address is found two machine words higher than that, at [RBP+8]