FAILURES ON PB2

Before this fix, the test suite had sporadic failures when
testing statistics related the the socket instruments.

A possible root cause is now identified:
in table_all_instr::rnd_next(), which is executed
when selecting from table EVENTS_WAITS_SUMMARY_BY_INSTANCE,
the code that returns a row for a socket waits is:

    case pos_all_instr::VIEW_SOCKET:
      {
        PFS_socket_iterator it=
global_socket_container.iterate(m_pos.m_index_2);
        socket= it.scan_next(& m_pos.m_index_2);
        if (file != NULL)
        {
          make_socket_row(socket);
          m_next_pos.set_after(&m_pos);
          return 0;
        }
      }
      break;
    }

The if statement is incorrect, and uses uninitialized memory.
It should be if (socket != NULL) instead.