lib: fix incorrect thread management

lib: fix incorrect thread management

The current code passes an address of a local variable to `thread_add_read`
which stores it into `thread->ref` by the lib. The next time the thread
callback is executed, the lib stores NULL into the `thread->ref` which
means it writes into some random memory on the stack.

To fix this, we should pass a pointer to the vector entry to the lib.

Signed-off-by: Igor Ryzhov <iryzhov@nfware.com>