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The patch looks good. I'd like the comment to say "We use urlsafe_b64decode here because some mailers apparently use the urlsafe b64 alphabet, and urlsafe_b64decode will correctly decode both the urlsafe and regular alphabets".
Also, the new header parser doesn't handle this case either, and furthermore it doesn't handle binascii errors at all (my comment in the code indicates I didn't think it could ever raise there). The fact that it doesn't handle the error at all can be considered a different issue, but it would be nice to add the same decode fix (and a test in test_email/test_headerregistry.py) for the new header parser. Here's one way to reproduce the issue:
>>> from email import message_from_string as mfs
>>> from email.policy import default
>>> m = mfs("From: =?iso-8859-1?B?QW5tZWxkdW5nIE5ldHphbnNjaGx1c3MgU_xkcmluZzNwLmpwZw==?=\n\n", policy=default)
>>> m['From']
Traceback (most recent call last):
 File "/home/rdmurray/python/p34/Lib/email/_encoded_words.py", line 109, in decode_b
 return base64.b64decode(padded_encoded, validate=True), defects
 File "/home/rdmurray/python/p34/Lib/base64.py", line 89, in b64decode
 raise binascii.Error('Non-base64 digit found')
binascii.Error: Non-base64 digit found
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/home/rdmurray/python/p34/Lib/email/message.py", line 391, in __getitem__
 return self.get(name)
 File "/home/rdmurray/python/p34/Lib/email/message.py", line 471, in get
 return self.policy.header_fetch_parse(k, v)
 File "/home/rdmurray/python/p34/Lib/email/policy.py", line 145, in header_fetch_parse
 return self.header_factory(name, ''.join(value.splitlines()))
 File "/home/rdmurray/python/p34/Lib/email/headerregistry.py", line 583, in __call__
 return self[name](name, value)
 File "/home/rdmurray/python/p34/Lib/email/headerregistry.py", line 194, in __new__
 cls.parse(value, kwds)
 File "/home/rdmurray/python/p34/Lib/email/headerregistry.py", line 334, in parse
 kwds['parse_tree'] = address_list = cls.value_parser(value)
 File "/home/rdmurray/python/p34/Lib/email/headerregistry.py", line 325, in value_parser
 address_list, value = parser.get_address_list(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 2313, in get_address_list
 token, value = get_address(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 2290, in get_address
 token, value = get_group(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 2246, in get_group
 token, value = get_display_name(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 2072, in get_display_name
 token, value = get_phrase(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 1747, in get_phrase
 token, value = get_word(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 1728, in get_word
 token, value = get_atom(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 1645, in get_atom
 token, value = get_encoded_word(value)
 File "/home/rdmurray/python/p34/Lib/email/_header_value_parser.py", line 1421, in get_encoded_word
 text, charset, lang, defects = _ew.decode('=?' + tok + '?=')
 File "/home/rdmurray/python/p34/Lib/email/_encoded_words.py", line 166, in decode
 bstring, defects = _cte_decoders[cte](bstring)
 File "/home/rdmurray/python/p34/Lib/email/_encoded_words.py", line 124, in decode_b
 raise AssertionError("unexpected binascii.Error")
AssertionError: unexpected binascii.Error