nemo-view.c: fix crash after some shortcuts

[jknockel]

On action_follow_symlink_callback() and
action_open_containing_folder_callback() we now confirm that exactly one thing is selected. Previously, if zero things were selected, we would segfault.

[mtwebster]

Hi, how do you reproduce the crash? These two menu items shouldn't be available if there's nothing selected (checked in real_update_menus).

[jknockel]

I found this crash originally when I had accidentally typed ctrl+alt+o instead of ctrl+alt+l trying to lock my screen, which triggered action_open_containing_folder_callback() on my focused nemo window. However, looking at the nemo code, I don't see a keyboard shortcut for action_follow_symlink_callback(), so I don't remember if I had originally added the check to that function purely defensively or if I had somehow found a way to trigger it.