Change source to be a unique_ptr.

The current code keeps a pointer to a local variable which doesn't
work too well. Change this to a unique_ptr and allocate the source
object that will be used instead.

Test: All unit tests pass.
Test: fastboot -w flashall on a mokey which crashed without this change.
Change-Id: Ief5437374181e514928c45dd540b42898901a137