Fix undocumented free() in x509_string_to_names()

Now programs/x509/cert_write san="DN:CN=#0000;DN:CN=#0000" is no longer
crashing with use-after-free, instead it's now failing cleanly:

 failed
  !  mbedtls_x509_string_to_names returned -0x2800 - X509 - Input invalid

That's better of course but still not great, will be fixed by future
commits.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>
diff --git a/include/mbedtls/x509.h b/include/mbedtls/x509.h
index 453f598..6b10461 100644
--- a/include/mbedtls/x509.h
+++ b/include/mbedtls/x509.h
@@ -332,7 +332,8 @@
  *                   call to mbedtls_asn1_free_named_data_list().
  *
  * \param[out] head  Address in which to store the pointer to the head of the
- *                   allocated list of mbedtls_x509_name
+ *                   allocated list of mbedtls_x509_name. Must point to NULL on
+ *                   entry.
  * \param[in] name   The string representation of a DN to convert
  *
  * \return           0 on success, or a negative error code.