ALSA: core: pcm_memory: dereference pointer after NULL checks

ALSA: core: pcm_memory: dereference pointer after NULL checks

Fix cppcheck warnings:

sound/core/pcm_memory.c:380:26: warning: Either the condition
'!substream' is redundant or there is possible null pointer
dereference: substream. [nullPointerRedundantCheck]
 struct snd_card *card = substream->pcm->card;
                         ^
sound/core/pcm_memory.c:384:6: note: Assuming that condition
'!substream' is not redundant
 if (PCM_RUNTIME_CHECK(substream))
     ^
sound/core/pcm_memory.c:380:26: note: Null pointer dereference
 struct snd_card *card = substream->pcm->card;
                         ^
sound/core/pcm_memory.c:433:26: warning: Either the condition
'!substream' is redundant or there is possible null pointer
dereference: substream. [nullPointerRedundantCheck]
 struct snd_card *card = substream->pcm->card;
                         ^
sound/core/pcm_memory.c:436:6: note: Assuming that condition
'!substream' is not redundant
 if (PCM_RUNTIME_CHECK(substream))
     ^
sound/core/pcm_memory.c:433:26: note: Null pointer dereference
 struct snd_card *card = substream->pcm->card;
                         ^

Signed-off-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200902212133.30964-4-pierre-louis.bossart@linux.intel.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>