lib: fix heap corruption in stream_fifo_free

lib: fix heap corruption in stream_fifo_free

When popping a stream from a stream_fifo, the stream->next pointer is
not NULL'd out. If this same stream is subsequently pushed onto a
stream_fifo (either the same one or a different one), because
stream_fifo's use tail insertion the ->next pointer is not updated and
thus will point to whatever the next stream in the first stream_fifo
was. stream_fifo_free does not check the count of the stream_fifo when
freeing its constituent elements, and instead walks the linked list.
Consequently it will continue walking into the first stream_fifo from
which the last stream was popped, freeing each stream contained there.
This leads to use-after-free errors.

This patch makes sure to set the ->next pointer to NULL when doing tail
insertion in stream_fifo_push and when popping a stream from a
stream_fifo.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>