commit ce40cd3f upstream.

Malicious (or egregiously buggy) userspace can trigger it, but it
should never happen in normal operation.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>