service: Fix null pointer dereference in CompleteFlush
service: Fix null pointer dereference in CompleteFlush

If the tracing session is destroyed in the middle of the PostTask,
CompleteFlush might end up dereferencing a null tracing_session.

Bug: https://github.com/google/perfetto/issues/75
Change-Id: Ia94697679e13e438e684f73acd204e92d640f7df
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto