gh-153569: keep token span invariants explicit

Author: pablogsal

Parser/lexer/lexer.h

@@ -17,6 +17,7 @@ _PyToken_TextView(const struct tok_state *tok, const struct token *token,
         *length = 0;
         return "";
     }
+    assert(_PyTok_SpanIsValid(token->span));
     assert(tok->buf != NULL);
     assert(tok->inp >= tok->buf);
     assert(token->span.start >= tok->buf_offset);