check_audit_mask(mysql_global_audit_mask, event_class_mask) is tested in
mysql_audit_general_log() and then assert in mysql_audit_acquire_plugins()
verifies that the condition still holds.
But this code path is not protected by LOCK_audit_mask, so
mysql_global_audit_mask can change its value between the if() and the
assert. That is, the assert is invalid and will fire if the
audit plugin is unloaded concurrently with mysql_audit_general_log().

Nothing bad will happen in this case though, we'll just do a useless
loop over all remaining installed audit plugins.

That is, the fix is simply to remove the assert.