gdbstub: fix compiler complaining

gdbstub: fix compiler complaining

    ./gdbstub.c: In function ‘handle_query_thread_extra’:
        /usr/include/glib-2.0/glib/glib-autocleanups.h:28:10:
    error: ‘cpu_name’ may be used uninitialized in this function
    [-Werror=maybe-uninitialized]
        g_free (*pp);
               ^
    ./gdbstub.c:2063:26: note: ‘cpu_name’ was declared here
        g_autofree char *cpu_name;
                         ^
    cc1: all warnings being treated as errors

Signed-off-by: Denis Plotnikov <dplotnikov@virtuozzo.com>
Message-Id: <20200326151407.25046-1-dplotnikov@virtuozzo.com>
Reported-by: Euler Robot <euler.robot@huawei.com>
Reported-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Miroslav Rezanina <mrezanin@redhat.com>
Message-Id: <20200325092137.24020-1-kuhn.chenqun@huawei.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20200403191150.863-7-alex.bennee@linaro.org>