Fix potential crash after unload of func_periodic_hook or test_message.

These modules save a pointer to the context they create on load, and
use that pointer to destroy the context at unload.  It is not safe
to save this pointer, it is replaced during load of pbx_config,
pbx_lua or pbx_ael.

This change causes the modules to pass NULL to ast_context_destroy,
a safer way to perform the unregistration since it does not use
a pointer that could become invalid.

ASTERISK-25085 #close
Reported by: Corey Farrell

Change-Id: I6a00ec8e38046058f97dc703e1adcde9bf517835