Default to wiki number 0 if diary is called outside a wiki buffer

If a diary page is opened from outside a wiki buffer the diary frequency
and start of week options are not respected as the wiki number is not
yet set for the local buffer.

Fix this by defaulting the wiki number to 0, which is what is done in
vimwiki#diary#make_note() for the diary_rel_path option that suffers
from the same problem.

Example on how to trigger the problem:

    let g:vimwiki_list = [{'path': '~/.vimwiki', 'diary_frequency': 'weekly', 'diary_start_week_day': 'monday'}]

    $ vim -c VimwikiMakeDiaryNote

Fixes: a3be479d5a ("Add a diary frequency option (#884)")
Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

autoload/vimwiki/diary.vim

@@ -37,6 +37,17 @@ endfunction
 
 function! vimwiki#diary#diary_date_link(...) abort
   " Return: <String> date
+
+  let wiki_nr = vimwiki#vars#get_bufferlocal('wiki_nr')
+  if wiki_nr < 0  " this happens when called outside a wiki buffer
+    let wiki_nr = 0
+  endif
+
+  if wiki_nr >= vimwiki#vars#number_of_wikis()
+    call vimwiki#u#error('Wiki '.wiki_nr.' is not registered in g:vimwiki_list!')
+    return
+  endif
+
   if a:0
     let l:timestamp = a:1
   else
@@ -56,10 +67,10 @@ function! vimwiki#diary#diary_date_link(...) abort
         \ 'friday': 5, 'saturday': 6,
         \ 'sunday': 0}
 
-  let l:frequency = vimwiki#vars#get_wikilocal('diary_frequency')
+  let l:frequency = vimwiki#vars#get_wikilocal('diary_frequency', wiki_nr)
 
   if l:frequency ==? 'weekly'
-    let l:start_week_day = vimwiki#vars#get_wikilocal('diary_start_week_day')
+    let l:start_week_day = vimwiki#vars#get_wikilocal('diary_start_week_day', wiki_nr)
     let l:weekday_num = str2nr(strftime('%w', l:timestamp))
     let l:days_to_end_of_week = (7-l:weekday_number[l:start_week_day]+weekday_num) % 7
     let l:computed_timestamp = l:timestamp