cpufreq: brcmstb-avs-cpufreq: add check for cpufreq_cpu_get's return value

cpufreq_cpu_get may return NULL. To avoid NULL-dereference check it
and return 0 in case of error.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: de322e08 ("cpufreq: brcmstb-avs-cpufreq: AVS CPUfreq driver for Broadcom STB SoCs")
Signed-off-by: Anastasia Belova <abelova@astralinux.ru>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>

drivers/cpufreq/brcmstb-avs-cpufreq.c

@@ -481,6 +481,8 @@ static bool brcm_avs_is_firmware_loaded(struct private_data *priv)
 static unsigned int brcm_avs_cpufreq_get(unsigned int cpu)
 {
 	struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
+	if (!policy)
+		return 0;
 	struct private_data *priv = policy->driver_data;
 
 	cpufreq_cpu_put(policy);