Error when creating a new account on expanded transaction

With the register in journal mode and you edit the account field so the
'Account x does not exist, do you want to create it' dialogue is shown
you get the following error...

gboolean boolean_from_key(const Account*, const std::vector<std::
__cxx11::basic_string<char> >&): assertion 'GNC_IS_ACCOUNT(acc)' failed

This was tracked down to a refresh being triggered with the account
dialogue so add a check for a valid account as part of the placeholder
test.

gnucash/register/ledger-core/split-register.c

@@ -1969,7 +1969,7 @@ gnc_split_register_get_account_by_name (SplitRegister *reg, BasicCell * bcell,
     g_free (account_name);
 
     /* See if the account (either old or new) is a placeholder. */
-    if (xaccAccountGetPlaceholder (account))
+    if (account && xaccAccountGetPlaceholder (account))
     {
         gnc_error_dialog (GTK_WINDOW (gnc_split_register_get_parent (reg)),
                           placeholder, name);