Duplicates in CiviPetition signatures cause errors

The contactId is never set in the case that a user is signing a petition but have already signed the petition, causing a nasty SQL error. To reproduce:

• Create a petition
• Sign the petition
• In a separate browser session (I had to actually use a separate browser, clearing session cookies didn't seem to work), sign the petition with information that will trigger a dedupe.

Expected result: "You have already signed this petition."
Actual result: CRM error.