Fix some minor Tramp problems

Fix some minor Tramp problems

* lisp/net/tramp-gvfs.el (tramp-gvfs-maybe-open-connection):
Check, that `tramp-password-save-function' is a function.

* lisp/net/tramp-smb.el (tramp-smb-handle-file-system-info):
Check, that there is a share.

* lisp/net/tramp.el (outline-regexp): Declare.
(tramp-get-debug-buffer): Let-bind `signal-hook-function'.

* test/lisp/net/tramp-tests.el (tramp-test04-substitute-in-file-name):
Skip some tests for Emacs 24 and 25; they let Emacs crash.