Backport of "qt: initialize GError properly in gst_qt_get_gl_wrapcontext()" into 1.20
Backport of !3327 (merged) "qt: initialize GError properly in gst_qt_get_gl_wrapcontext()".
Commits:
- edc2e831 qt: initialize GError properly in gst_qt_get_gl_wrapcontext()