Handle empty gvectors when doubling size.

Closes PR 14843.

pkgs/data-pkgs/data-lib/data/gvector.rkt

@@ -46,7 +46,7 @@
              (vector-set! v index item))
            (set-gvector-n! gv (+ n item-count))]
           [else
-           (let* ([nn (let loop ([nn n])
+           (let* ([nn (let loop ([nn (max n 1)])
                         (if (<= (+ n item-count) nn) nn (loop (* 2 nn))))]
                   [nv (make-vector nn #f)])
              (vector-copy! nv 0 v)

pkgs/data-pkgs/data-test/tests/data/gvector.rkt

@@ -22,6 +22,13 @@
                 gv))
              '#(1 2 3))
 
+(test-equal? "gvector-add! (lots)"
+             (let ()
+               (define g (gvector))
+               (apply gvector-add! g (build-list 11 values))
+               (gvector->vector g))
+             (for/vector ([i 11]) i))
+
 (test-equal? "gvector-ref"
              (let ([gv (gvector 1 2 3)])
                ;; 3 valid refs + 1 not-found