handle_wrap: fix NULL pointer dereference

Fix a NULL pointer dereference in src/handle_wrap.cc which is really a
use-after-close bug.

The test checks that unref() after close() works on process.stdout but
this bug affects everything that derives from HandleWrap. I discovered
it because child processes would sometimes quit for no reason (that is,
no reason until I turned on core dumps.)

This is a back-port of commit ccd3722 from the v0.10 branch.
test: make stdout-close-unref work in test runner

process.stdout isn't fully initialized yet by the time the test starts
when invoked with `python tools/test.py`. Use process.stdin instead and
force initialization with process.stdin.resume().

This is a back-port of commit 2e70dda from the v0.10 branch.