pyglib_error_check: Re-add missing NULL check

Commit adcfe96d accidentally dropped the check if *error is NULL, i. e.
any error is actually set. Due to that, pyglib_error_check() always returned
TRUE. Reintroduce the check.

Thanks to Alberto Mardegan for spotting this!

https://bugzilla.gnome.org/show_bug.cgi?id=669415