Fix test expectation in gmp/tests/bug74670.phpt

No changes to the input required, because the test actually is intended to
verify the behavior for a missing `}`, it's just that the report position changed.

Author: TimWolla

ext/gmp/tests/bug74670.phpt

@@ -8,5 +8,5 @@ $str = 'C:3:"GMP":4:{s:6666666666:""}';
 var_dump(unserialize($str));
 ?>
 --EXPECTF--
-Warning: unserialize(): Error at offset 13 of 29 bytes in %s on line %d
+Warning: unserialize(): Error at offset 17 of 29 bytes in %s on line %d
 bool(false)