Tighten up obs_local_part checks further to avoid IndexError

Also, reformat slightly to make the indentation more obvious

Author: fsc-eriker

Lib/email/_header_value_parser.py

@@ -1517,14 +1517,16 @@ def get_obs_local_part(value):
         obs_local_part.defects.append(errors.InvalidHeaderDefect(
             "abandoned parse; truncated value?"))
     else:
-        if (obs_local_part[0].token_type == 'dot' or
-            obs_local_part[0].token_type=='cfws' and
-            obs_local_part[1].token_type=='dot'):
+        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')):
             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
-            obs_local_part[-2].token_type=='dot'):
+        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')):
             obs_local_part.defects.append(errors.InvalidHeaderDefect(
                 "Invalid trailing '.' in local part"))
     if obs_local_part.defects: