mem: Provide better diagnostic for unconnected port
mem: Provide better diagnostic for unconnected port

When _masterPort is null, a message to that effect is
more helpful than a segfault.

hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5,hoangt/tpzsimul.gem5