Silent corruption due to shared working directory #102

I have discovered a bug that sometimes causes data corruption on both compression and recompression, and as far as I can tell pfverify doesn't catch it.

The problem is related to multiple instances of precomp running in parallel using the same working directory, I'm suspecting the instances are overwriting each other's temporary files and this is causing corruption.

I have worked-around the problem by creating a temporary directory for each instance to use as the working directory.

I am running this on Ubuntu Linux.