ALSA: compress_offload: dereference after checking for NULL pointer

Fix cppcheck warning and only dereference once the initial checks are
done:

sound/core/compress_offload.c:516:38: warning: Either the condition
'!stream' is redundant or there is possible null pointer dereference:
stream. [nullPointerRedundantCheck]
 struct snd_compr_runtime *runtime = stream->runtime;
                                     ^
sound/core/compress_offload.c:518:17: note: Assuming that condition
'!stream' is not redundant
if (snd_BUG_ON(!(stream) || !(stream)->runtime))
                ^
sound/core/compress_offload.c:516:38: note: Null pointer dereference
 struct snd_compr_runtime *runtime = stream->runtime;
                                     ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Link: https://lore.kernel.org/r/20200902212133.30964-7-pierre-louis.bossart@linux.intel.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>