Bug 792763 - deleting a transaction can cause a SIGSEGV

Prevent a NULL transaction from crashing xaccTransCountSplits and
prevent gnc_plugin_business_update_menus from calling it with a
NULL transaction.

gnucash/gnome/gnc-plugin-business.c

@@ -923,7 +923,7 @@ gnc_plugin_business_update_menus (GncPluginPage *plugin_page)
     if (is_txn_register)
     {
         Transaction *trans = gnc_plugin_page_register_get_current_txn (GNC_PLUGIN_PAGE_REGISTER(plugin_page));
-        if (xaccTransCountSplits(trans) > 0)
+        if (trans && xaccTransCountSplits(trans) > 0)
             is_bus_txn = (xaccTransGetFirstAPARAcctSplit(trans, TRUE) != NULL);
         is_bus_doc = (xaccTransGetTxnType (trans) == TXN_TYPE_INVOICE);
     }

libgnucash/engine/Transaction.c

@@ -2292,6 +2292,7 @@ int
 xaccTransCountSplits (const Transaction *trans)
 {
     gint i = 0;
+    g_return_val_if_fail (trans != NULL, 0);
     FOR_EACH_SPLIT(trans, i++);
     return i;
 }