intel/common/block: Fix issue found by klockwork

src/soc/intel/common/block/pmc/pmclib.c
  Function acpi_get_sleep_type: Pointer ps checked for
  NULL may be dereferenced.

BRANCH=None
TEST=Built & booted Yorp board.

Change-Id: I15fe39fd9f930be56d03c2ffe62fb6f17249d4b5
Signed-off-by: John Zhao <john.zhao@intel.com>
Reviewed-on: https://review.coreboot.org/29149
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/soc/intel/common/block/pmc/pmclib.c

@@ -95,9 +95,13 @@ __weak void pmc_soc_restore_power_failure(void)
 int acpi_get_sleep_type(void)
 {
 	struct chipset_power_state *ps;
+	int prev_sleep_state = ACPI_S0;
 
 	ps = pmc_get_power_state();
-	return ps->prev_sleep_state;
+	if (ps)
+		prev_sleep_state = ps->prev_sleep_state;
+
+	return prev_sleep_state;
 }
 
 static uint32_t pmc_reset_smi_status(void)