Fix bug in mbedtls_asn1_store_named_data()

When passed a zero-length val, the function was free-ing the buffer as
the documentation suggests:

 * \param val_len   The minimum length of the data buffer needed.
 *                  If this is 0, do not allocate a buffer for the associated
 *                  data.
 *                  If the OID was already present, enlarge, shrink or free
 *                  the existing buffer to fit \p val_len.

However it kept the previous length, leaving the val structure in the
corresponding item in the output list in an inconsistent state:

    p == NULL but len != 0

As a result, functions that would try using this item in the list
(including the same function!) afterwards would trip an dereference the
NULL pointer.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

library/asn1write.c

@@ -412,6 +412,7 @@ mbedtls_asn1_named_data *mbedtls_asn1_store_named_data(
     } else if (val_len == 0) {
         mbedtls_free(cur->val.p);
         cur->val.p = NULL;
+        cur->val.len = 0;
     } else if (cur->val.len != val_len) {
         /*
          * Enlarge existing value buffer if needed