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 <guptarud@gmail.com>
Closes: https://lore.kernel.org/lkml/692cfe9a-8c05-4ce4-813e-82b3f310019a@gmail.com/
Reviewed-by: Konrad Dybcio <konradybcio@kernel.org>
Tested-by: Rudraksha Gupta <guptarud@gmail.com>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Stephan Gerhold <stephan.gerhold@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Kuldeep Singh <quic_kuldsing@quicinc.com>
Link: https://lore.kernel.org/r/20240930083328.17904-1-brgl@bgdev.pl
Signed-off-by: Bjorn Andersson <andersson@kernel.org>

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)