Fix base64-codec and bz2-codec incremental decoders

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-08-19.11:52:00.605>
labels = ['type-bug', 'library']
title = 'Fix base64-codec and bz2-codec incremental decoders'
updated_at = <Date 2016-08-19.11:52:00.605>
user = 'https://github.com/vadmium'

bugs.python.org fields:

activity = <Date 2016-08-19.11:52:00.605>
actor = 'martin.panter'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-08-19.11:52:00.605>
creator = 'martin.panter'
dependencies = []
files = ['44149']
hgrepos = []
issue_num = 27799
keywords = ['patch']
message_count = 1.0
messages = ['273106']
nosy_count = 1.0
nosy_names = ['martin.panter']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue27799'
versions = ['Python 2.7', 'Python 3.5', 'Python 3.6']