full_trace: Fix seg fault if WORKLOAD is not defined.
full_trace: Fix seg fault if WORKLOAD is not defined.

getenv() returns NULL if the environment variable is not defined.
A std::string created using a NULL pointer results in a seg fault.

Change-Id: Icbda0a3da9a64bb87ac74b2da82fee4eb5395742

rgly/LLVM-Tracer,rgly/LLVM-Tracer,rgly/LLVM-Tracer