QMP command dumpdtb crash bug

[Markus Armbruster <armbru@redhat.com>]

Watch this:

    $ gdb --args ../qemu/bld/qemu-system-aarch64 -S -M virt -display none -qmp stdio
    [...]
    (gdb) r
    [...]
    {"QMP": {"version": {"qemu": {"micro": 50, "minor": 2, "major": 7}, "package": "v7.2.0-2331-gda89f78a7d"}, "capabilities": ["oob"]}}
    [New Thread 0x7fffed62c6c0 (LWP 1021967)]
    {"execute": "qmp_capabilities", "arguments": {"enable": ["oob"]}}
    {"return": {}}
    {"execute": "dumpdtb", "arguments": {"filename": "fdt.dtb"}}

    Thread 1 "qemu-system-aar" received signal SIGSEGV, Segmentation fault.
    qmp_dumpdtb (filename=0x5555581c5170 "fdt.dtb", errp=errp@entry=0x7fffffffdae8)
        at ../softmmu/device_tree.c:661
    661	    size = fdt_totalsize(current_machine->fdt);

current_machine->fdt is non-null here.  The crash is within
fdt_totalsize().

I suspect ...

    void qmp_dumpdtb(const char *filename, Error **errp)
    {
        g_autoptr(GError) err = NULL;
        uint32_t size;

        if (!current_machine->fdt) {
            error_setg(errp, "This machine doesn't have a FDT");
            return;
        }

... we're missing an "FDT isn't ready" guard here.

        size = fdt_totalsize(current_machine->fdt);

        g_assert(size > 0);

        if (!g_file_set_contents(filename, current_machine->fdt, size, &err)) {
            error_setg(errp, "Error saving FDT to file %s: %s",
                       filename, err->message);
        }
    }

Also, I think the error message "does not have a FDT" should say "an
FDT".