[3.8] bpo-37764: Fix infinite loop when parsing unstructured email headers. (GH-15239...

[3.8] bpo-37764: Fix infinite loop when parsing unstructured email headers. (GH-15239) (GH-15686)

Fixes a case in which email._header_value_parser.get_unstructured hangs the system for some invalid headers. This covers the cases in which the header contains either:
- a case without trailing whitespace
- an invalid encoded word

https://bugs.python.org/issue37764

This fix should also be backported to 3.7 and 3.8

https://bugs.python.org/issue37764
(cherry picked from commit c5b242f87f31286ad38991bc3868cf4cfbf2b681)

Co-authored-by: Ashwin Ramaswami <aramaswamis@gmail.com>