LeakSanitizer warns memory leaks

[hongxuchen]

At https://github.com/jnovy/pxz/blob/master/pxz.c#L338, the allocated memory is not freed. The problem is that when there are many files to be processed at once (e.g., ./pxz -k -T 8 -q -f c-input/*, and since the maximum argument length is 131072 on my machine, c.f. here), it may eat up memories -- the allocations are inside a loop traversing the input files. Since each time the leaked memory is 25169920 on my machine, 25169920*131072=3.2990718e+12bytes, which is roughly 3200Gigabytes.
When the command lasts for a long time, it may also slowdown the host machine.

Also, as seen in #31, there is no upper bound limit for the specified thread number, this may lead to quick segfault.

[jnovy]

This is now fixed and merged to master.