gstgiosrc: Don't leak scheme string in gst_gio_src_query()

Add a g_free() in the code path that forwards the query to the parent
handler.

Part-of: <gstreamer/gst-plugins-base!1084>
33 jobs for !1084 with backport-mr-1078-into-1.18-gio-dont-leak-scheme in 25 minutes and 33 seconds (queued for 3 seconds)
merge request