monitor: fix memory leak in monitor_fdset_dup_fd_find_remove

monitor: fix memory leak in monitor_fdset_dup_fd_find_remove

When remove dup_fd in monitor_fdset_dup_fd_find_remove function,
we need to free mon_fdset_fd_dup. ASAN shows memory leak stack:

Direct leak of 96 byte(s) in 3 object(s) allocated from:
    #0 0xfffd37b033b3 in __interceptor_calloc (/lib64/libasan.so.4+0xd33b3)
    #1 0xfffd375c71cb in g_malloc0 (/lib64/libglib-2.0.so.0+0x571cb)
    #2 0xaaae25bf1c17 in monitor_fdset_dup_fd_add /qemu/monitor/misc.c:1724
    #3 0xaaae265cfd8f in qemu_open /qemu/util/osdep.c:315
    #4 0xaaae264e2b2b in qmp_chardev_open_file_source /qemu/chardev/char-fd.c:122
    #5 0xaaae264e47cf in qmp_chardev_open_file /qemu/chardev/char-file.c:81
    #6 0xaaae264e118b in qemu_char_open /qemu/chardev/char.c:237
    #7 0xaaae264e118b in qemu_chardev_new /qemu/chardev/char.c:964
    #8 0xaaae264e1543 in qemu_chr_new_from_opts /qemu/chardev/char.c:680
    #9 0xaaae25e12e0f in chardev_init_func /qemu/vl.c:2083
    #10 0xaaae26603823 in qemu_opts_foreach /qemu/util/qemu-option.c:1170
    #11 0xaaae258c9787 in main /qemu/vl.c:4089
    #12 0xfffd35b80b9f in __libc_start_main (/lib64/libc.so.6+0x20b9f)
    #13 0xaaae258d7b63  (/qemu/build/aarch64-softmmu/qemu-system-aarch64+0x8b7b63)

Reported-by: Euler Robot <euler.robot@huawei.com>
Signed-off-by: Chen Qun <kuhn.chenqun@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20200115072016.167252-1-kuhn.chenqun@huawei.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>