mem: Fix MemChecker unique_ptr type mismatch

mem: Fix MemChecker unique_ptr type mismatch

This patch fixes the type of the unique_ptr instances, to ensure that
the data that is allocated with new[] is also deleted with
delete[]. The issue was highlighted by ASAN.

Change-Id: I2c5510424959d862a9954d83e728d901bb18d309
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Stephan Diestelhorst <stephan.diestelhorst@arm.com>