BUG/MINOR: mux-spop: Fix null-pointer deref on SPOP stream allocation failure

BUG/MINOR: mux-spop: Fix null-pointer deref on SPOP stream allocation failure

When we try to allocate a new SPOP stream, if an error is encountered,
spop_strm_destroy() is called to released the eventually allocated
stream. But, it must only be called if a stream was allocated. If the
reported error is an SPOP stream allocation failure, we must just leave to
avoid null-pointer dereference.

This patch should fix point 1 of the issue #2993. It must be backported as
far as 3.1.