Fix infinite loop in email folding logic (GH-12732)

As far as I can tell, this infinite loop would be triggered if:

1. The value being folded contains a single word (no spaces) longer than
   max_line_length
2. The max_line_length is shorter than the encoding's name + 9
   characters.

bpo-36564: https://bugs.python.org/issue36564

Lib/email/_header_value_parser.py

@@ -2846,15 +2846,22 @@ def _fold_as_ew(to_encode, lines, maxlen, last_ew, ew_combine_allowed, charset):
         trailing_wsp = to_encode[-1]
         to_encode = to_encode[:-1]
     new_last_ew = len(lines[-1]) if last_ew is None else last_ew
+
+    encode_as = 'utf-8' if charset == 'us-ascii' else charset
+
+    # The RFC2047 chrome takes up 7 characters plus the length
+    # of the charset name.
+    chrome_len = len(encode_as) + 7
+
+    if (chrome_len + 1) >= maxlen:
+        raise errors.HeaderParseError(
+            "max_line_length is too small to fit an encoded word")
+
     while to_encode:
         remaining_space = maxlen - len(lines[-1])
-        # The RFC2047 chrome takes up 7 characters plus the length
-        # of the charset name.
-        encode_as = 'utf-8' if charset == 'us-ascii' else charset
-        text_space = remaining_space - len(encode_as) - 7
+        text_space = remaining_space - chrome_len
         if text_space <= 0:
             lines.append(' ')
-            # XXX We'll get an infinite loop here if maxlen is <= 7
             continue
 
         to_encode_word = to_encode[:text_space]

Lib/email/parser.py

@@ -13,7 +13,6 @@ from email.feedparser import FeedParser, BytesFeedParser
 from email._policybase import compat32
 
 
-
 class Parser:
     def __init__(self, _class=None, *, policy=compat32):
         """Parser of RFC 2822 and MIME email messages.