Fix crash when PL server stops
Fix crash when PL server stops

Xilinx/PYNQ,schelleg/PYNQ,yunqu/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,Xilinx/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,yunqu/PYNQ,cathalmccabe/PYNQ,Xilinx/PYNQ,cathalmccabe/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,cathalmccabe/PYNQ,schelleg/PYNQ,yunqu/PYNQ,Xilinx/PYNQ,Xilinx/PYNQ,yunqu/PYNQ