GeezerVison

LOAD_SYMBOL() returns null when no backend library is loaded or the
symbol cannot be resolved. af_get_last_error called the resulting
function pointer unconditionally, which is an indirect call through a
null pointer.

Every other unified entry point goes through CALL(), which null-checks
and returns AF_ERR_LOAD_LIB. af_get_last_error returns void and cannot,
so report an empty error instead.

Fixes #3713

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_012SRueobKykUocB1SRdHsEE

Every caller passes len as NULL and formats the message with %s, so a NULL string is undefined behaviour. Allocate an empty message the way the C API does.