BGE: Fix issues with async libload.

This patch fixes:
- the call of LibFree on a unfinished loaded library;
- memory leak created on end of game : the async libraries are loaded but not converted, so not freed with the master scene.

Reviewers: campbellbarton, sybren, youle, hg1, moguri, lordloki

Reviewed By: moguri, lordloki

Differential Revision: https://developer.blender.org/D1571

doc/python_api/rst/bge_types/bge.types.KX_LibLoadStatus.rst

@@ -25,6 +25,12 @@ base class --- :class:`PyObjectPlus`
 
       :type: callable
 
+   .. attribute:: finished
+
+      The current status of the lib load.
+
+      :type: boolean
+
    .. attribute:: progress
 
       The current progress of the lib load as a normalized value from 0.0 to 1.0.