qcacld-3.0: Fix potential NULL pointer dereference

Add check to ensure context pointer is not NULL before dereferencing

Change-Id: I737ae4d1e5aa07bcbbe612a93905a23fa857b90a
CRs-fixed: 2669214

core/sap/src/sap_module.c

@@ -3004,6 +3004,11 @@ qdf_freq_t wlansap_get_chan_band_restrict(struct sap_context *sap_ctx)
 		return 0;
 
 	mac = cds_get_context(QDF_MODULE_ID_PE);
+	if (!mac) {
+		sap_err("Invalid MAC context");
+		return 0;
+	}
+
 	if (ucfg_reg_get_curr_band(mac->pdev, &band) != QDF_STATUS_SUCCESS) {
 		sap_err("Failed to get current band config");
 		return 0;