Initialize the variable `index_hint`

Initialize the variable `index_hint`

Prior to this commit, `index_hint` is initialized if `Context && !attach_msg`.

If `query_quadoption(C_Quit, _("Quit NeoMutt?")) == MUTT_YES`,
`!Context || ((check = mx_mbox_close(&Context)) == 0)` is `false` and
`(check == MUTT_NEW_MAIL) || (check == MUTT_REOPENED)` the function `update_index`
is called with parameter `index_hint`.

As one of the condition is `query_quadoption(C_Quit, _("Quit NeoMutt?")) == MUTT_YES`,
the real current position is probably not so important.