'The Contact Sub Type does not match the Contact type for this record' when saving a new contact

Clicking 'Save and New' when editing a contact which has one or more subtypes - the variable $this->_contactSubType is appended to the URL. However, in this flow that variable is delimited w/ ASCII 01 character, which results in the behavior described below.

(see line 1041 in CRM_Contact_Form_Contact::postProcess)

Fix this by:

• check length of $this->_contactSubType list (can be 0, 1 or more since contacts may have multiple subtypes)
• If $this->_contactSubType list length = 1, trim out the delimiters and append to the URL as we do now
• If $this->_contactSubType list length > 1, do not append to URL (we do not support loading of Add Contact form with multiple subtypes passed via URL).

---- Original Post ----
Steps to reproduce:

• Create a new Contact sub Type 'Student' of 'Individual'
• Create a new Contact with the new sub Type and save.
• From the view page, edit the same contact and use "Save and New"

The URL of this last page will have:

http://<domain>/civicrm/contact/add?reset=1&ct=Individual&cst=%01Student%01

and when attempting to Save generates the error.

Edit: after some extra digging, I discovered that all the 'Student' contacts in the database had `contact_sub_type`='%01Student%01' (urlencoded values added for clarity).
Still, the civicrm_contact_type table has only 'Student'.