datalog: tokenizer raises on unterminated block comment (249/249)

Bug: `/* unclosed` was silently consumed to EOF, swallowing any
Datalog code that followed inside the (never-closing) comment.
Programs would produce empty parses with no error.

Fix: skip-block-comment! now raises when it hits EOF without
finding `*/`. Error message includes the position where the
problem was first detected. Line comments (`%`) and properly
closed block comments (`/* ... */`) are unaffected.

1 new tokenize test verifies the error path.

lib/datalog/tokenizer.sx

@@ -63,7 +63,9 @@
         (fn
           ()
           (cond
-            ((>= pos src-len) nil)
+            ((>= pos src-len)
+              (error (str "Tokenizer: unterminated block comment "
+                          "(started at position " pos ")")))
             ((and (= (cur) "*") (< (+ pos 1) src-len) (= (dl-peek 1) "/"))
               (advance! 2))
             (else (do (advance! 1) (skip-block-comment!))))))