No thread number upper bound leads to crash

Since several occurrences of memory allocation `malloc_safe` are dependent on thread number (`-T`), but there is no upper bound limit for thread number, this may lead to segfault. An example is like:

```
./pxz -k -T 2000 -q -f /tmp/test.in
[1]    72116 segmentation fault  ./pxz -k -T 2000 -q -f /tmp/test.in
```

Where /tmp/test.in is an empty file.