core: fix segv when poping last element

Signed-off-by: Marc-Antoine Perennou <Marc-Antoine@Perennou.com>
Release GPaste 3.2.2

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

* 3.0:
  Release GPaste 3.2.2
  core: fix segv when poping last element

Conflicts:
	NEWS
	README.md
	configure.ac
	libgpaste/core/gpaste-clipboards-manager.c
	libgpaste/core/gpaste-history.c