startup writers: Release unique_ptr/lock correctly in ReturnTraceWriter
startup writers: Release unique_ptr/lock correctly in ReturnTraceWriter

Another oversight.. sigh :P

TBR: aa8b77df5d887ec30b6fa9e5e60a727a5eeba345@google.com
Change-Id: I84cba5fdf081da1ec44cbdbc218fd8a7498910ce

google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto