add guard to current_account_changed against nullptr parameters

- current_account_changed may get a null account as parameter. I have
no clue this can lead to a problem other than from this function
using it. So this patch attend to fix the uses of a null account in
current_account_changed.

Change-Id: I02f8f1caf8462b86e55479555203bd9137c90d29
1 file changed