Only close stdin/stdout/stderr for debug malloc.

The debug malloc code unconditionally closes stdin/stdout/stderr,
which means that other atexit functions cannot use them. Only
close these if there is a debug malloc final function to call.

This doesn't appear to be a problem on most normal applications or the
atexit_exit bionic unit test would be failing. However, if you
enable stat dumping in jemalloc, nothing prints. Most likely trying
to add an atexit function from within libc is causing that atexit
to run after the debug malloc atexit function.

Change-Id: I963720d4ccaaa511e44af07a7461f17eb3f84e8e