ContactDetailActivity: guard against null pointer

If the starredMenuItem doesn't exist (say, because it was never inflated),
onPrepareOptionsMenu() still tries to set its OnMenuItemClickListener,
which causes a NullPointerException and kills the whole People app.

Normally, the starredMenuItem *is* inflated so this can't happen.
However... see my next commit.
Remove the star menu item from contact detail view