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

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

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