Prevent using filter() on invalid types of values

Closes #75.

autoload/startify.vim

@@ -263,10 +263,13 @@ endfunction
 
 " Function: s:show_dir {{{1
 function! s:show_dir(cnt) abort
+  if empty(v:oldfiles)
+    return a:cnt
+  endif
+
   let cnt     = a:cnt
   let num     = s:numfiles
   let entries = {}
-
   let cwd     = getcwd()
   let files   = filter(map(copy(v:oldfiles), 'resolve(fnamemodify(v:val, ":p"))'), 'match(v:val, cwd) == 0')
 
@@ -308,15 +311,18 @@ endfunction
 
 " Function: s:show_files {{{1
 function! s:show_files(cnt) abort
-  let cnt     = a:cnt
-  let num     = s:numfiles
-  let entries = {}
+  if empty(v:oldfiles)
+    return a:cnt
+  endif
 
-  if !empty(v:oldfiles)
   if exists('s:last_message')
     call s:print_section_header()
   endif
 
+  let cnt     = a:cnt
+  let num     = s:numfiles
+  let entries = {}
+
   for fname in v:oldfiles
     let fullpath = resolve(fnamemodify(fname, ':p'))
 
@@ -345,7 +351,6 @@ function! s:show_files(cnt) abort
   call append('$', '')
 
   return cnt
-  endif
 endfunction
 
 " Function: s:show_sessions {{{1