[report-utilities] use fold for gnc:accounts-count-splits

previous definition would build intermediate lists for each step. also
handles the null account-list.

gnucash/report/report-utilities.scm

@@ -741,7 +741,7 @@ query instead.")
 
 ;; function to count the total number of splits to be iterated
 (define (gnc:accounts-count-splits accounts)
-  (apply + (map length (map xaccAccountGetSplitList accounts))))
+  (fold (lambda (a b) (+ b (length (xaccAccountGetSplitList a)))) 0 accounts))
 
 ;; Sums up any splits of a certain type affecting a set of accounts.
 ;; the type is an alist '((str "match me") (cased #f) (regexp #f))

gnucash/report/test/test-report-utilities.scm

@@ -477,6 +477,10 @@
         44
         (gnc:accounts-count-splits (list expense income)))
 
+      (test-equal "gnc:accounts-count-splits null"
+        0
+        (gnc:accounts-count-splits '()))
+
       (let ((account-balances (gnc:get-assoc-account-balances
                                (list bank gbp-bank)
                                (lambda (acct)