Skip unusable paths in __fish_print_hostnames

See
https://github.com/fish-shell/fish-shell/issues/4511#issuecomment-343022740.

This would try to `cd` to ~/.ssh even if it didn't exist. That's
clearly bogus.

share/functions/__fish_print_hostnames.fish

@@ -62,6 +62,10 @@ function __fish_print_hostnames -d "Print a list of known hostnames"
                 end
             end
 
+            # Skip unusable paths.
+            test -d "$relative_path" -a -x "$relative_path"
+            or return
+
             builtin cd $relative_path
             set -l new_paths
             for path in $paths