softmmu/device_tree: Silence compiler warning with --enable-sanitizers

softmmu/device_tree: Silence compiler warning with --enable-sanitizers

If I configure my build with --enable-sanitizers, my GCC (v8.5.0)
complains:

.../softmmu/device_tree.c: In function ‘qemu_fdt_add_path’:
.../softmmu/device_tree.c:560:18: error: ‘retval’ may be used uninitialized
 in this function [-Werror=maybe-uninitialized]
     int namelen, retval;
                  ^~~~~~

It's a false warning since the while loop is always executed at least
once (p has to be non-NULL, otherwise the derefence in the if-statement
earlier will crash). Thus let's switch to a do-while loop here instead
to make the compiler happy in all cases.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Yanan Wang <wangyanan55@huawei.com>
Message-id: 20220107133844.145039-1-thuth@redhat.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>