hantek-6xxx: fix potential NULL dereference

Check pointers' validity before dereferencing.

This was reported by clang's scan-build.

src/hardware/hantek-6xxx/api.c

@@ -460,6 +460,8 @@ static int config_list(uint32_t key, GVariant **data,
 			*data = std_gvar_array_u32(ARRAY_AND_SIZE(devopts_cg));
 			break;
 		case SR_CONF_COUPLING:
+			if (!devc)
+				return SR_ERR_ARG;
 			*data = g_variant_new_strv(devc->coupling_vals, devc->coupling_tab_size);
 			break;
 		case SR_CONF_VDIV: