Check for regressions caused by accesslib updates

After the accesslib changes in MDL-29602, a number of regressions have been found.

In 2.2, get_context_instance(xxxxx, CONTEXT_USER) can now return false in some cases (like if the user is deleted).

However, some old code always assumes that a context is returned eg:

 $usercontextid = get_context_instance(CONTEXT_USER, $userid)->id;

Someone must check all the calls to get_context_instance(xxxxx, CONTEXT_USER) and make sure they are handling the case of a returned "false" correctly, eg for the above:

 $usercontext = get_context_instance(CONTEXT_USER, $userid);

 if (!$usercontext) {

    continue;

 }