9p: fix leak in synth_name_to_path()

Leak found thanks to ASAN:

Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x55995789ac90 in __interceptor_malloc (/home/elmarco/src/qemu/build/x86_64-softmmu/qemu-system-x86_64+0x1510c90)
    #1 0x7f0a91190f0c in g_malloc /home/elmarco/src/gnome/glib/builddir/../glib/gmem.c:94
    #2 0x5599580a281c in v9fs_path_copy /home/elmarco/src/qemu/hw/9pfs/9p.c:196:17
    #3 0x559958f9ec5d in coroutine_trampoline /home/elmarco/src/qemu/util/coroutine-ucontext.c:116:9
    #4 0x7f0a8766ebbf  (/lib64/libc.so.6+0x50bbf)
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: NGreg Kurz <groug@kaod.org>

hw/9pfs/9p-synth.c

@@ -495,6 +495,7 @@ static int synth_name_to_path(FsContext *ctx, V9fsPath *dir_path,
     }
 out:
     /* Copy the node pointer to fid */
+    g_free(target->data);
     target->data = g_memdup(&node, sizeof(void *));
     target->size = sizeof(void *);
     return 0;