nemo-tree-sidebar.c: Fix context menu handling for invalid rows.

[MiQ71]

Fix context menu handling in the tree sidebar when there is no valid file associated with the target row.

This fixes two existing crash cases:

• Nemo crashes on the right-click menu of the Tree view of the sidebar #3782: Right-clicking the empty area below the tree entries opened the selection context menu without a valid popup_file. Selecting Properties could then crash Nemo.
• Nemo crash on right click empty label in tree view #3640: Right-clicking the (Empty) placeholder of an empty directory provides a tree path, but no corresponding NemoFile, which could also lead to a crash.

While testing the fix, I also found that invoking the context menu from the keyboard (Shift+F10) could crash when the tree view had focus but no valid row was selected. In this case the popup-menu signal calls popup_menu() without a GdkEventButton, while update_menu_states() unconditionally dereferenced the event.

The change:

• uses the mouse position for pointer-triggered context menus;
• uses the current tree cursor for keyboard-triggered context menus;
• checks that the tree path resolves to a valid NemoFile;
• only opens the context menu when a valid target was found.

Tested with a local build of Nemo 6.7.6:

• right-click on a normal tree entry: context menu works;
• Properties on a normal entry: works;
• right-click below the tree entries: no menu, no crash;
• right-click on an (Empty) placeholder: no menu, no crash;
• Shift+F10 on a valid tree entry: context menu works;
• Shift+F10 without a valid tree entry: no crash.