syntax: fix guard for multiple pattern variables under ...

Fix the part of `syntax`/`datum` that determines whether an exception
handler is needed to convert an error from `map` to one from
`syntax`/`datum`.

pkgs/racket-test-core/tests/racket/macro.rktl

@@ -743,6 +743,16 @@
 (syntax-test #'(datum-case '(1 "x" -> y) (->) [(a b -> c) (define q 1)])
              #rx"macro.rktl:.*no expression after a sequence of internal definitions")
 
+(let ()
+  (define-syntax-rule (check-error clause)
+    (err/rt-test
+     (datum-case '((1 2) (3)) () clause)
+     (lambda (exn)
+       (and (regexp-match? #rx"incompatible ellipsis match counts for template"
+                           (exn-message exn))))))
+  (check-error [((a ...) (b ...)) (datum ((a b) ...))])
+  (check-error [((a ...) (b ...)) (datum ((a 0 b) ...))])
+  (check-error [((a ...) (b ...)) (datum (((a) (b)) ...))]))
 
 ;; ----------------------------------------
 ;; Check `#%variable-reference' expansion to make sure

racket/collects/racket/private/sc.rkt

@@ -1136,7 +1136,7 @@
               [(and (syntax? p)
                     (prefab-struct-key (syntax-e p)))
                (loop (cdr (vector->list (struct->vector (syntax-e p)))) use-ellipsis? depth found)]
-              [else #f])))
+              [else found])))
 
   (-define (no-ellipses? stx)
            (cond