firmware: qcom: scm: fix a NULL-pointer dereference

Some SCM calls can be invoked with __scm being NULL (the driver may not
have been and will not be probed as there's no SCM entry in device-tree).
Make sure we don't dereference a NULL pointer.

Fixes: 449d0d8 ("firmware: qcom: scm: smc: switch to using the SCM allocator")
Reported-by: Rudraksha Gupta <[email protected]>
Closes: https://lore.kernel.org/lkml/[email protected]/
Reviewed-by: Konrad Dybcio <[email protected]>
Tested-by: Rudraksha Gupta <[email protected]>
Reviewed-by: Dmitry Baryshkov <[email protected]>
Reviewed-by: Stephan Gerhold <[email protected]>
Signed-off-by: Bartosz Golaszewski <[email protected]>
Reviewed-by: Kuldeep Singh <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Bjorn Andersson <[email protected]>

Author: Bartosz Golaszewski

drivers/firmware/qcom/qcom_scm.c

@@ -216,7 +216,7 @@ static DEFINE_SPINLOCK(scm_query_lock);
 
 struct qcom_tzmem_pool *qcom_scm_get_tzmem_pool(void)
 {
-	return __scm->mempool;
+	return __scm ? __scm->mempool : NULL;
 }
 
 static enum qcom_scm_convention __get_convention(void)