Fix NullPointerException in KernelCpuThreadReaders.

If KernelCpuThreadReader init failed, then the null instance was passed
to KernelCpuThreadReaderDiff, which then failed when invoked.

Test: Build only (bug is difficult to reproduce).
Bug: 150988957

Change-Id: Id46d466b931d816dda85c4f068a4bec53c6f3032