gi: Check Gst has not been initialized before loading bindings

It can have been initialized by some C code (in a C app with plugins
for example).

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=788088

gi/overrides/Gst.py

@@ -633,4 +633,5 @@ def deinit():
 
 Gst.deinit = deinit
 
-deinit_pygst()
+if not Gst.is_initialized():
+    deinit_pygst()