Fix: null check after deref, use uninitialized or freed variable

Fix: null check after deref, use uninitialized or freed variable

Found by Coverity:

CID 1376179 (#1 of 1): Dereference before null check
(REVERSE_INULL)check_after_deref: Null-checking stream_state suggests
that it may be null, but it has already been dereferenced on all paths
leading to the check.

Reorganized this function so we clear the stream_state (set to NULL)
whenever we return success or error, so the caller don't end up using
uninitialized variable.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>