Add 'strict pointer mode' to BlendFile

Add a 'strict pointer mode' to the `BlendFile` class, which is enabled
by default. This allows users of the `BlendFile` class to decide whether
a bad pointer (i.e. one that points to a non-existing datablock) returns
`None` or raises a `SegmentationFault` exception.

tests/test_blendfile_loading.py

@@ -217,6 +217,15 @@ class PointerTest(AbstractBlendFileTest):
         with self.assertRaises(exceptions.SegmentationFault):
             scene.get_pointer(b"ed")
 
+    def test_disabled_strict_pointer_mode(self):
+        scene = self.bf.code_index[b"SC"][0]
+        ed_ptr = scene.get(b"ed")
+        del self.bf.block_from_addr[ed_ptr]
+
+        self.bf.strict_pointer_mode = False
+        dereferenced = scene.get_pointer(b"ed")
+        self.assertIsNone(dereferenced)
+
     def test_abs_offset(self):
         scene = self.bf.code_index[b"SC"][0]
         ed = scene.get_pointer(b"ed")