ALSA: hda: Fix possible null-ptr-deref when assigning a stream

In the Linux kernel, the following vulnerability has been resolved: ALSA: hda: Fix possible null-ptr-deref when assigning a stream While AudioDSP drivers assign streams exclusively of HOST or LINK type, nothing blocks a user to attempt to assign a COUPLED stream. As supplied substream instance may be a stub, what is the case when code-loading, such scenario ends with null-ptr-deref.