[job-report] Prevent Crash if there is no AR account

I think this crash is triggered because the 'account' variable
defaults to the first available AR account. If there's no AR account
it becomes null, and querying null's default book leads to segfault.

I guess I can fix segfault too by fixing gnc_account_get_book.

gnucash/report/business-reports/job-report.scm

@@ -541,7 +541,7 @@
 	 (end-date (gnc:time64-end-day-time 
 		       (gnc:date-option-absolute-time
 			(opt-val gnc:pagename-general (N_ "To")))))
-	 (book (gnc-account-get-book account))
+	 (book (gnc-get-current-book))
          (date-format (gnc:options-fancy-date book))
 	 (type (opt-val "__reg" "owner-type"))
 	 (type-str "")

libgnucash/engine/Account.cpp

@@ -1015,6 +1015,7 @@ xaccInitAccount (Account * acc, QofBook *book)
 QofBook *
 gnc_account_get_book(const Account *account)
 {
+    if (!account) return NULL;
     return qof_instance_get_book(QOF_INSTANCE(account));
 }