Extend safety to include portability issues

[r-ross]

The tutorial associates safety with not having C undefined behaviours, but consider extending that to other portability issues as described in ISO/IEC 9899:1999 Annex J. For example, a program relying on the values of padding bytes is relying on unspecified behavior instead of undefined behavior but is not really safe. Some of the listed portability issues would be very difficult to check so no one should expect completeness, but finding as many as reasonably possible can help avoid scenarios where "it works for me" and "it works for the rest of the team" yet "it blows up in the customer's face."