[1.6.x] Fixed #21662 -- Kept parent reference in prepared geometry

Thanks Robert Scott for the report.
Backport of 542198c1d0 from master.

django/contrib/gis/geos/prepared.py

@@ -11,7 +11,12 @@ class PreparedGeometry(GEOSBase):
     ptr_type = capi.PREPGEOM_PTR
 
     def __init__(self, geom):
-        if not isinstance(geom, GEOSGeometry): raise TypeError
+        # Keeping a reference to the original geometry object to prevent it
+        # from being garbage collected which could then crash the prepared one
+        # See #21662
+        self._base_geom = geom
+        if not isinstance(geom, GEOSGeometry):
+            raise TypeError
         self.ptr = capi.geos_prepare(geom.ptr)
 
     def __del__(self):

django/contrib/gis/geos/tests/test_geos.py

@@ -1043,6 +1043,10 @@ class GEOSTest(unittest.TestCase, TestDataMixin):
             self.assertEqual(mpoly.intersects(pnt), prep.intersects(pnt))
             self.assertEqual(c, prep.covers(pnt))
 
+        # Original geometry deletion should not crash the prepared one (#21662)
+        del mpoly
+        self.assertTrue(prep.covers(Point(5, 5)))
+
     def test_line_merge(self):
         "Testing line merge support"
         ref_geoms = (fromstr('LINESTRING(1 1, 1 1, 3 3)'),