Credit goes to palladian
The project doesn't match the currently available xv6 source code, which already has this project implemented in it!
palladian tracked down a different xv6 source from the Github page of a U of Wisconsin student. We had to edit the Makefile to find the QEMU executable correctly. We added null.c to the user folder (also edited makefile.mk there), which demonstrates the lack of memory safety.
Start with the code in start.zip. Extract it and run make clean and make qemu-nox. Then inside the xv6 system run null to see the lack of safety! If you want to compare the results of null with the actual machine code, you can run objdump -d user/null.o.
You might have to manually run make clean and make qemu-nox every time you make a change to the code.