[FIX] Lab3 grader: a bug caused by stack garbage and ending "\0"

Problem description:

• There was a bug in our Lab3 grader. If your lab3 log has "[ERROR] concurrency bug", you might be affected.
• The bug is caused by garbage data on stack plus improper string handling.
• In particular, we forgot to explicitly add "\0" to strings from your dbservers. Meanwhile, if stack happens to have garbage data immediately following the string, the grader would (wrongly) think the garbage is part of returned value.
• The bug happened with a probability of ~10% (this is the probability of happening on one workload.)

Next step and the fix:

• We will regrade your Lab3. (stay tuned for when we're done)
• The grader is posted on Canvas homepage. You can download it and run on your end.
• A one-line bug fix can be found on "line 236" of "grader.c" (you can also search for "WASABUG" to see it).