Call _exit() instead of exit() on child failure.

As explained on man _exit:

"The  function  _exit()  terminates  the calling process "immediately".
Any open file descriptors belonging to the process are closed;"

If exit() is called instead, the resulting zip file will be corrupted.

BUG: 27900023
BUG: 27804637
Change-Id: I6591749603753d6016f350c629b7082cf6780ae2