Don't process malformed file names

I'm not sure what the cause was, but I ended up with an entry in my file
list that was just some regex pattern. So the processing in this loop
was crashing at the `glob` below. Fix this by ignoring non-existant
files.

Author: lanza

autoload/startify.vim

@@ -654,6 +654,10 @@ function! s:filter_oldfiles_unsafe(path_prefix, path_format, use_env) abort
       break
     endif
 
+    if !filereadable(fname)
+      continue
+    endif
+
     if s:is_in_skiplist(fname)
       " https://github.com/mhinz/vim-startify/issues/353
       continue