Better error for _EnterBstrWrapped on allocated (indef) strings (#221)

Better error for QCBORDecode_EnterBstrWrapped on allocated strings. Since indefinite-length strings are always allocated, this gives a better error for attempts to use this on indefinite-length strings.

The TODO: to make this work is closed out as it was decided not to support QCBORDecode_EnterBstrWrapped() on indefinite-length strings. To do so would require increasing the size of QCBORItem.

* Add limitations comment

---------

Co-authored-by: Laurence Lundblade <lgl@securitytheory.com>
diff --git a/inc/qcbor/qcbor_encode.h b/inc/qcbor/qcbor_encode.h
index 75b4d64..774f272 100644
--- a/inc/qcbor/qcbor_encode.h
+++ b/inc/qcbor/qcbor_encode.h
@@ -366,6 +366,7 @@
  * - Tags on labels are ignored during decoding.
  * - The maximum tag nesting is @c QCBOR_MAX_TAGS_PER_ITEM (typically 4).
  * - Works only on 32- and 64-bit CPUs.
+ * - QCBORDecode_EnterBstrWrapped() doesn't work on indefinite-length strings.
  *
  * The public interface uses @c size_t for all lengths. Internally the
  * implementation uses 32-bit lengths by design to use less memory and