Fix: Memory leak on error paths of relay_add_stream

Fix: Memory leak on error paths of relay_add_stream

On error paths the memory allocated for stream is never freed.

Also, fix undefined behavior on asprintf alloc failure. According to
asprintf(3), the content of the pointer passed to it is undefined if an
alloc failure occurs, so we could end up freeing a pointer in an
undefined state. Force its value to NULL.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>