Malformed header causes message import failure

[rpcross]

Example:
...
From: RFC Errata System rfc-editor@rfc-editor.org
Cc: example@gmail.com,
Content-Type: text/plain; charset=UTF-8
...

The Cc: value with a trailing comma causes a failure.

To Reproduce:
NO_REFOLD_POLICY = policy.SMTP.clone(refold_source='none')
with open('path', 'rb') as f:
data = f.read()
msg = email.message_from_bytes(data, policy=NO_REFOLD_POLICY)
msg.items()
...
File "/usr/lib64/python3.6/email/_header_value_parser.py", line 1365, in get_phrase
token, value = get_word(value)
File "/usr/lib64/python3.6/email/_header_value_parser.py", line 1340, in get_word
if value[0]=='"':
IndexError: string index out of range

Interestingly this doesn't happen when not specifying policy.