twrpTar: fix segfault after encrypted backups

also use unsigned int for core_count instead of unsigned long long.
I'll change it back when 4-billion-core devices are common.

PS2
- cast return value via intptr_t (may be important for 64 bit platforms)
- output errors from TWFunc::Wait_For_Child to console

Change-Id: I04158daa0b64e44d68e179d626a83d81cf5d49f7