lib: Do not blindly set the _read and _write pointers to NULL

lib: Do not blindly set the _read and _write pointers to NULL

Adding a read with the address of the thread pointer we want to
use will allow lib/thread.c to properly handle your thread pointers.
Instead we were setting the pointer to NULL before we passed
into the _read and _write thread functions.  Remove the NULL
pointer set and just let thread.c handle everything.

vty_stdio_resume and vty_read would blindly add read and write
which would cause vty_event() to drop the thread pointer.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>