Fix stringp error when inserting new snippets #852

New snippets (created with yas-new-snippet) may not yet be associated
with a file when it is passed to the functions in yas-prompt-functions.
One of those functions, +snippets-prompt-private, prioritizes private
snippets over default ones, but determines which is which with
`file-in-directory-p`, which expects string arguments.

To fix this, we can safely assume that if a snippet has no associated
file, it is a private snippet.

modules/feature/snippets/autoload/snippets.el

@@ -7,8 +7,9 @@ are multiple choices."
   (when-let*
       ((choices
         (or (cl-loop for tpl in choices
-                     if (file-in-directory-p (yas--template-get-file tpl)
-                                             +snippets-dir)
+                     for file = (yas--template-get-file tpl)
+                     if (or (null file)
+                            (file-in-directory-p file +snippets-dir))
                      collect tpl)
             choices)))
     (if (cdr choices)