syntax/parse: add dynamic check for null ellipsis-head match

Otherwise, nullable ellipsis-head patterns could cause infinite loop.

racket/collects/syntax/parse/private/parse.rkt

@@ -984,8 +984,9 @@ Conventions:
                    (for/list ([id (in-list ids)]) (bound-id-table-ref table id)))])
            (with-syntax ([(id ...) ids]
                          [(alt-id ...) alt-ids])
-             #`(let ([alt-id (rep:combine repc (attribute id) alt-id)] ...)
-                 k))))
+             #`(let ([alt-id (rep:combine repc (attribute id) alt-id)] ...
+                     [diff (ps-difference pr pr*)])
+                 (if (zero? diff) (error/null-eh-match) k)))))
        (syntax-case #'repc ()
          [#f #`(parse:H x cx x* cx* pr* head pr es k*)]
          [_  #`(parse:H x cx x* cx* pr* head pr es

racket/collects/syntax/parse/private/residual.rkt

@@ -60,6 +60,7 @@
          datum->syntax/with-clause
          check/force-syntax-list^depth
          check-literal*
+         error/null-eh-match
          begin-for-syntax/once
 
          name->too-few/once
@@ -216,6 +217,10 @@
              (and used-phase (- used-phase mod-phase)))
      ctx id)))
 
+;; error/null-eh-match : -> (escapes)
+(define (error/null-eh-match)
+  (error 'syntax-parse "an ellipsis-head pattern matched an empty sequence"))
+
 ;; (begin-for-syntax/once expr/phase1 ...)
 ;; evaluates in pass 2 of module/intdefs expansion
 (define-syntax (begin-for-syntax/once stx)