NautilusFile: fix crash in modify_link_hash_table

We crash in g_hash_table_lookup_extended if the file's symlink_name is
set but the file is not a symlink, since the hash function (g_str_hash)
does not allow NULL values but target_uri is NULL.

https://bugzilla.gnome.org/show_bug.cgi?id=711583
nautilus-window: Check for valid location entry widget

https://bugzilla.gnome.org/show_bug.cgi?id=722616