Fix: relayd: Explicit null dereferenced

Fix: relayd: Explicit null dereferenced

Coverity warns about an explicit null dereference on an early exit path.

Coverity report:
  CID 1405577 (#1 of 1): Explicit null dereferenced (FORWARD_NULL)7.
  var_deref_model: Passing null pointer previous_stream_fd to
  stream_fd_put, which dereferences it.

Reported-by: Coverity (1405577) Explicit null dereferenced
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>