malloc/tst-mallocfork2: Kill lingering process for unexpected failures

If the test fails due some unexpected failure after the children
creation, either in the signal handler by calling abort or in the main
loop; the created children might not be killed properly.

This patches fixes it by:

  * Avoid aborting in the signal handler by setting a flag that
    an error has occured and add a check in the main loop.

  * Add a atexit handler to handle kill child processes.

Checked on x86_64-linux-gnu.