formatter: tabnr expects a number bot got List

Make sure, that the second argument is actually a number and not a list.
Note: even when fixing this, the tabpage still shows some strange output
e.g. currently
 1   1.%{len(tabpagebuflist(1))} 

In a vim window. Not sure, why Vim does no longer evaluate those %{..}
sections. Perhaps some change in Vim?

closes #2616

autoload/airline/extensions/tabline/formatters/tabnr.vim

@@ -4,12 +4,14 @@
 scriptencoding utf-8
 
 function! airline#extensions#tabline#formatters#tabnr#format(tab_nr_type, nr)
+  " TODO: Is this actually useful? Or should we remove this one?
+  let nr = type(a:nr) == type([]) ? a:nr[0] : a:nr
   let spc=g:airline_symbols.space
   if a:tab_nr_type == 0 " nr of splits
-    return spc. '%{len(tabpagebuflist('.a:nr.'))}'
+    return spc. '%{len(tabpagebuflist('.nr.'))}'
   elseif a:tab_nr_type == 1 " tab number
-    return spc. a:nr
+    return spc. nr
   else "== 2 splits and tab number
-    return spc. a:nr. '.%{len(tabpagebuflist('.a:nr.'))}'
+    return spc. nr. '.%{len(tabpagebuflist('.nr.'))}'
   endif
 endfunction