Ensure package-pinned-packages is bound before referencing it

Ensure package-pinned-packages is bound before referencing it

Add a bound-and-true-p guard to package-pinned-packages before
referencing it in use-package-ensure-elpa.

Package pinning was introduced in Emacs 24.4, and hence
package-pinned-packages in unbound by default in earlier versions.

Relevant commits:
	 72452b5 Merge pull request https://github.com/jwiegley/use-package/issues/367 from ketbra/master
	 5053f75 Make pin and ensure compatible

Fixes https://github.com/jwiegley/use-package/issues/375

Copyright-paperwork-exempt: yes