libpdx: Fix ThreadLocalBufferTest.InitialValue unittest

A call to GetBuffer() has been inadvertently removed during code
upstreaming to master. See the BUG below for more details.

Bug: 36456321
Test: `pdx_tests` now succeed on Sailfish
Change-Id: Ic7d6ecbbd9b0e59e74b48a66a9e3701dc44695bc