Fix p2p_segwit.py test

[Gnappuraz]

Since there is tail update in additional_bytes, if additional_bytes % 55 = 0, it will not add the +1 byte needed to pass assert_equal(vsize, MAX_BLOCK_BASE_SIZE + 1).
The algorithm needs to do one last pass, that will happen if additional_bytes % 55 = 0, where it will add min(0+1, 55) extra_bytes. If additional_bytes % 55 > 0, after the update additional_bytes will be < 0 and skip the last pass.
This has been discovered because the coinbase scriptSig changed from 71 to 72 bytes (due to changed coinbase reward in #794), causing the total block size to be % 55 with no remainder.

[castarco]

utACK 7133b3a

[Ruteri]

utACK 7133b3a

looked at it in more detail

[scravy]

utACK 1ac84d0