org-agenda.el: Fix bug in `org-agenda-get-day-entries'
* org-agenda.el (org-agenda-get-day-entries): Set the agenda
buffer inconditionnally.
For example, when `org-agenda-get-day-entries' is called
through ̀org-diary', we need to make sure `org-agenda-buffer'
is set, otherwise org-diary triggers an error.