res_stir_shaken.c: Fix crash when stir_shaken.conf is invalid

res_stir_shaken.c: Fix crash when stir_shaken.conf is invalid

* If the call to ast_config_load() returns CONFIG_STATUS_FILEINVALID,
check_for_old_config() now returns LOAD_DECLINE instead of continuing
on with a bad pointer.

* If CONFIG_STATUS_FILEMISSING is returned, check_for_old_config()
assumes the config is being loaded from realtime and now returns
LOAD_SUCCESS.  If it's actually not being loaded from realtime,
sorcery will catch that later on.

* Also refactored the error handling in load_module() a bit.

Resolves: #884