Validate first digit of Belgian VAT number

Thanks to antonio-ginestar find pointing this out.

Closes #488

Author: arthurdejong

stdnum/be/vat.py

@@ -1,6 +1,6 @@
 # vat.py - functions for handling Belgian VAT numbers
 #
-# Copyright (C) 2012-2016 Arthur de Jong
+# Copyright (C) 2012-2025 Arthur de Jong
 #
 # This library is free software; you can redistribute it and/or
 # modify it under the terms of the GNU Lesser General Public
@@ -67,6 +67,8 @@ def validate(number: str) -> str:
         raise InvalidFormat()
     if len(number) != 10:
         raise InvalidLength()
+    if not number[0] in '01':
+        raise InvalidComponent()
     if checksum(number) != 0:
         raise InvalidChecksum()
     return number

tests/test_be_vat.doctest

@@ -1,6 +1,6 @@
 test_be_vat.doctest - more detailed doctests for stdnum.be.vat module
 
-Copyright (C) 2020 Arthur de Jong
+Copyright (C) 2020-2025 Arthur de Jong
 
 This library is free software; you can redistribute it and/or
 modify it under the terms of the GNU Lesser General Public
@@ -31,7 +31,15 @@ Tests for corner cases.
 Traceback (most recent call last):
     ...
 InvalidFormat: ...
+>>> vat.validate('BE000000000')
+Traceback (most recent call last):
+    ...
+InvalidFormat: ...
 >>> vat.validate('000000')
 Traceback (most recent call last):
     ...
 InvalidFormat: ...
+>>> vat.validate('BE2000021323')
+Traceback (most recent call last):
+    ...
+InvalidComponent: ...