Return early if $a->user is absent in BaseModule

- Address https://github.com/friendica/friendica/issues/8877#issuecomment-660266026

src/BaseModule.php

@@ -140,6 +140,10 @@ abstract class BaseModule
 			return false;
 		}
 
+		if (empty($a->user)) {
+			return false;
+		}
+
 		$sec_hash = hash('whirlpool', $a->user['guid'] . $a->user['prvkey'] . session_id() . $x[0] . $typename);
 
 		return ($sec_hash == $x[1]);