Introduce _handle_bad_field to make subclassing easier

Many extra fields (e.g., 0x000a and 0x000c) have sub-tags using the
same (id, len, *fields) format.  Introduce a `_handle_bad_field` class
method to allow a subclass to parse with same logic while providing a
customized error message.

Author: danny0838

Lib/zipfile/__init__.py

@@ -198,6 +198,10 @@ class LargeZipFile(Exception):
 class _Extra:
     FIELD_STRUCT = struct.Struct('<HH')
 
+    @classmethod
+    def _handle_bad_field(cls, xid, xlen):
+        raise BadZipFile("Corrupt extra field %04x (size=%d)" % (xid, xlen))
+
     @classmethod
     def iter(cls, data, validate=False):
         """Iter through and yield each (field, id)."""
@@ -213,8 +217,7 @@ def iter(cls, data, validate=False):
                 xid, xlen = None, 0
             else:
                 if validate and pos + 4 + xlen > data_len:
-                    raise BadZipFile(
-                        "Corrupt extra field %04x (size=%d)" % (xid, xlen))
+                    cls._handle_bad_field(xid, xlen)
             yield data[pos:pos + 4 + xlen], xid
             pos += 4 + xlen