Wrap BillTermDecRef and TaxTableDecRef in test for shutdown

When Gnucash is shutting down, the TaxTables and BillTerms will be
destroyed/freed and may already have been so there is no point trying
to decrement a reference that is used to stop them being destroyed if
in use.

libgnucash/engine/gncCustomer.c

@@ -358,11 +358,12 @@ static void gncCustomerFree (GncCustomer *cust)
     g_list_free (cust->jobs);
     g_free (cust->balance);
 
-    if (cust->terms)
-        gncBillTermDecRef (cust->terms);
-    if (cust->taxtable)
+    if (!qof_book_shutting_down (qof_instance_get_book (QOF_INSTANCE(cust))))
     {
-        gncTaxTableDecRef (cust->taxtable);
+        if (cust->terms)
+            gncBillTermDecRef (cust->terms);
+        if (cust->taxtable)
+            gncTaxTableDecRef (cust->taxtable);
     }
 
     /* qof_instance_release (&cust->inst); */

libgnucash/engine/gncEntry.c

@@ -462,10 +462,14 @@ static void gncEntryFree (GncEntry *entry)
         gncAccountValueDestroy (entry->i_tax_values);
     if (entry->b_tax_values)
         gncAccountValueDestroy (entry->b_tax_values);
-    if (entry->i_tax_table)
-        gncTaxTableDecRef (entry->i_tax_table);
-    if (entry->b_tax_table)
-        gncTaxTableDecRef (entry->b_tax_table);
+
+    if (!qof_book_shutting_down (qof_instance_get_book (QOF_INSTANCE(entry))))
+    {
+        if (entry->i_tax_table)
+            gncTaxTableDecRef (entry->i_tax_table);
+        if (entry->b_tax_table)
+            gncTaxTableDecRef (entry->b_tax_table);
+    }
 
     /* qof_instance_release (&entry->inst); */
     g_object_unref (entry);

libgnucash/engine/gncInvoice.c

@@ -423,10 +423,14 @@ static void gncInvoiceFree (GncInvoice *invoice)
     g_list_free (invoice->entries);
     g_list_free (invoice->prices);
 
-    if (invoice->printname) g_free (invoice->printname);
+    if (invoice->printname)
+        g_free (invoice->printname);
 
-    if (invoice->terms)
-        gncBillTermDecRef (invoice->terms);
+    if (!qof_book_shutting_down (qof_instance_get_book (QOF_INSTANCE(invoice))))
+    {
+        if (invoice->terms)
+            gncBillTermDecRef (invoice->terms);
+    }
 
     /* qof_instance_release (&invoice->inst); */
     g_object_unref (invoice);

libgnucash/engine/gncVendor.c

@@ -501,10 +501,13 @@ static void gncVendorFree (GncVendor *vendor)
     g_list_free (vendor->jobs);
     g_free (vendor->balance);
 
-    if (vendor->terms)
-        gncBillTermDecRef (vendor->terms);
-    if (vendor->taxtable)
-        gncTaxTableDecRef (vendor->taxtable);
+    if (!qof_book_shutting_down (qof_instance_get_book (QOF_INSTANCE(vendor))))
+    {
+        if (vendor->terms)
+            gncBillTermDecRef (vendor->terms);
+        if (vendor->taxtable)
+            gncTaxTableDecRef (vendor->taxtable);
+    }
 
     /* qof_instance_release (&vendor->inst); */
     g_object_unref (vendor);