paste-and-pop: add guards

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>

Conflicts:
	libgpaste/keybinder/gpaste-paste-and-pop-keybinding.c
fix previous commit

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Merge branch '3.0'

* 3.0:
  fix previous commit
  paste-and-pop: add guards

Conflicts:
	libgpaste/keybinder/gpaste-paste-and-pop-keybinding.c