nir/tests: Initialize nir_serialize_test member dup.

Fix defect reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR) uninit_member: Non-static class member dup is not initialized in this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee vlee@freedesktop.org

cc: @anholt @karolherbst