python:tests: Catch strings passed to utf16_encoded_len() with embedded nulls

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>

python/samba/tests/ndr/gkdi.py

@@ -30,6 +30,9 @@ import samba.tests
 def utf16_encoded_len(s: str) -> int:
     """Return the number of bytes required to encode a string as null‐terminated
     UTF‐16."""
+    if "\x00" in s:
+        raise ValueError("string contains an embedded null")
+
     return len(s.encode("utf-16-le")) + 2