Calendar: Prevent NPE

* File.list() can return null (i.e. due to an I/O error) resulting in a
  NPE when trying to get the length
* Fix logic to prevent this
* No single-line statement due to the still possible I/O error if calling
  list() twice

BUGBASH-91
Change-Id: Ieeb92a592031315030c5d7aaf4a99b61b345df1d
(cherry picked from commit 7315b87c3b03b1e2198d614ddce140a0901564cb)
(cherry picked from commit bc4acc4)