Update for pythonGH-104802: don't register dot defect if there is a single cwf

Don't add new error cases when refactoring; simply ensure that the code avoids an IndexError for the cases it attempts to handle

Author: fsc-eriker

Lib/email/_header_value_parser.py

@@ -1519,14 +1519,14 @@ def get_obs_local_part(value):
     else:
         if (obs_local_part[0].token_type == 'dot'
             or (obs_local_part[0].token_type=='cfws'
-                and len(obs_local_part) == 1
-                or obs_local_part[1].token_type=='dot')):
+                and len(obs_local_part) > 1
+                and obs_local_part[1].token_type=='dot')):
             obs_local_part.defects.append(errors.InvalidHeaderDefect(
                 "Invalid leading '.' in local part"))
         if (obs_local_part[-1].token_type == 'dot'
             or (obs_local_part[-1].token_type=='cfws'
-                and len(obs_local_part) == 1
-                or obs_local_part[-2].token_type=='dot')):
+                and len(obs_local_part) > 1
+                and obs_local_part[-2].token_type=='dot')):
             obs_local_part.defects.append(errors.InvalidHeaderDefect(
                 "Invalid trailing '.' in local part"))
     if obs_local_part.defects: