Fix some uninitialized variable.

Description

Fix some uninitialized variable in the new routine for exchange periodic systems. This addresses partly #289 (closed). However, there is another error reported by valgrind that I cannot make sense of it at the moment.

News snippet

Fix some uninitialized variable.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

Contributes to #289 (closed)