bpo-29990: Fix range checking in GB18030 decoder (#1495)

When decoding a 4-byte GB18030 sequence, the first and third byte cannot exceed 0xFE.

Lib/test/test_codecencodings_cn.py

@@ -48,6 +48,12 @@ class Test_GB18030(multibytecodec_support.TestBase, unittest.TestCase):
         (b"abc\x84\x32\x80\x80def", "replace", 'abc\ufffd2\ufffd\ufffddef'),
         (b"abc\x81\x30\x81\x30def", "strict", 'abc\x80def'),
         (b"abc\x86\x30\x81\x30def", "replace", 'abc\ufffd0\ufffd0def'),
+        # issue29990
+        (b"\xff\x30\x81\x30", "strict", None),
+        (b"\x81\x30\xff\x30", "strict", None),
+        (b"abc\x81\x39\xff\x39\xc1\xc4", "replace", "abc\ufffd\x39\ufffd\x39\u804a"),
+        (b"abc\xab\x36\xff\x30def", "replace", 'abc\ufffd\x36\ufffd\x30def'),
+        (b"abc\xbf\x38\xff\x32\xc1\xc4", "ignore",  "abc\x38\x32\u804a"),
     )
     has_iso10646 = True