unix: fix processing process handles queue

Make sure we initialize it after the handle was removed from the pending
queue so that QUEUE_REMOVE doesn't do an invalid write when the process
is closed.

Valgrind output:

==4362== Invalid write of size 8
==4362==    at 0x407DB8: uv__process_close (process.c:515)
==4362==    by 0x404F94: uv_close (core.c:138)
==4362==    by 0x4037C5: main (invalid_write.c:33)
==4362==  Address 0xffeffc820 is not stack'd, malloc'd or (recently) free'd
==4362==
==4362== Invalid write of size 8
==4362==    at 0x407DC3: uv__process_close (process.c:515)
==4362==    by 0x404F94: uv_close (core.c:138)
==4362==    by 0x4037C5: main (invalid_write.c:33)
==4362==  Address 0xffeffc828 is not stack'd, malloc'd or (recently) free'd

Refs: joyent/libuv/issues/1584