hw/nios2:fix leak of fdevice tree blob

The device tree blob returned by load_device_tree is malloced.
We should free it after cpu_physical_memory_write().
Reported-by: NEuler Robot <euler.robot@huawei.com>
Signed-off-by: NChen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: NLaurent Vivier <laurent@vivier.eu>
Message-Id: <20200218091154.21696-2-kuhn.chenqun@huawei.com>
Signed-off-by: NLaurent Vivier <laurent@vivier.eu>

hw/nios2/boot.c

@@ -109,6 +109,7 @@ static int nios2_load_dtb(struct nios2_boot_info bi, const uint32_t ramsize,
     }
 
     cpu_physical_memory_write(bi.fdt, fdt, fdt_size);
+    g_free(fdt);
     return fdt_size;
 }