bpo-29412: Fix indexError when parsing a header value ending unexpectedly (GH-14387)

* patched string index out of range error in get_word function of _header_value_parser.py and created tests in test__header_value_parser.py for CFWS.
* Raise HeaderParseError instead of continuing when parsing a word.

Lib/email/_header_value_parser.py

@@ -1360,6 +1360,9 @@ def get_word(value):
         leader, value = get_cfws(value)
     else:
         leader = None
+    if not value:
+        raise errors.HeaderParseError(
+            "Expected 'atom' or 'quoted-string' but found nothing.")
     if value[0]=='"':
         token, value = get_quoted_string(value)
     elif value[0] in SPECIALS: