Fix T51877: Deleting a scene uses freed memory

At the moment libblock_remap_data_preprocess is using
FOREACH_SCENE_OBJECT to iterate over all the objects of the scene and
unlink them.

However we were storing a reference to the Base of the removed object.
Anyways, the loop is now sanitized so that this crash no longer happens.

Also now we have an unittest for this.

tests/python/render_layer/test_scene_delete.py

@@ -0,0 +1,39 @@
+# ############################################################
+# Importing - Same For All Render Layer Tests
+# ############################################################
+
+import unittest
+import os
+import sys
+
+from render_layer_common import *
+
+
+# ############################################################
+# Testing
+# ############################################################
+
+class UnitTesting(RenderLayerTesting):
+    def test_scene_delete(self):
+        """
+        See if a scene can be properly deleted
+        """
+        import bpy
+
+        scene = bpy.context.scene
+        bpy.data.scenes.new('New')
+        bpy.data.scenes.remove(scene)
+
+
+# ############################################################
+# Main - Same For All Render Layer Tests
+# ############################################################
+
+if __name__ == '__main__':
+    import sys
+
+    extra_arguments = sys.argv[sys.argv.index("--") + 1:] if "--" in sys.argv else []
+    sys.argv = [__file__] + (sys.argv[sys.argv.index("--") + 2:] if "--" in sys.argv else [])
+
+    UnitTesting._extra_arguments = extra_arguments
+    unittest.main()