formats_err: add nav_parent_multi_one_unused

See kaitai-io/kaitai_struct#961

formats_err/nav_parent_multi_one_unused.ksy

@@ -0,0 +1,23 @@
+# nav_parent_multi_one_unused.ksy: /types/common/seq/0/size:
+# 	error: don't know how to call method 'len_body' of object type 'CalcKaitaiStructType(false)'
+#
+meta:
+  id: nav_parent_multi_one_unused
+seq:
+  - id: len_body
+    type: u1
+  - id: branch
+    type: common
+types:
+  # This type is unreachable from the top-level `seq`, but it is still
+  # generated, so it must affect the derived type of `_parent` in the
+  # `common` type, otherwise we would run into
+  # https://github.com/kaitai-io/kaitai_struct/issues/961.
+  unreachable:
+    seq:
+      - id: branch
+        type: common
+  common:
+    seq:
+      - id: body
+        size: _parent.len_body