Fix an off by one index error in makeseeds.py

Summary: Each line of the dnsseed.dump contains at least 12 items so we assert the length then access indexes 0 to 11. The assertion is off by one.

Test Plan: Run the makeseeds.py against a dnsseed.dump that is missing the user agent string and check it doesn't crash.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16089