vvfat: fix ubsan issue in create_long_filename

vvfat: fix ubsan issue in create_long_filename

Found with test sbsaref introduced in [1].

[1] https://patchew.org/QEMU/20241203213629.2482806-1-pierrick.bouvier@linaro.org/

../block/vvfat.c:433:24: runtime error: index 14 out of bounds for type 'uint8_t [11]'
    #0 0x56151a66b93a in create_long_filename ../block/vvfat.c:433
    #1 0x56151a66f3d7 in create_short_and_long_name ../block/vvfat.c:725
    #2 0x56151a670403 in read_directory ../block/vvfat.c:804
    #3 0x56151a674432 in init_directories ../block/vvfat.c:964
    #4 0x56151a67867b in vvfat_open ../block/vvfat.c:1258
    #5 0x56151a3b8e19 in bdrv_open_driver ../block.c:1660
    #6 0x56151a3bb666 in bdrv_open_common ../block.c:1985
    #7 0x56151a3cadb9 in bdrv_open_inherit ../block.c:4153
    #8 0x56151a3c8850 in bdrv_open_child_bs ../block.c:3731
    #9 0x56151a3ca832 in bdrv_open_inherit ../block.c:4098
    #10 0x56151a3cbe40 in bdrv_open ../block.c:4248
    #11 0x56151a46344f in blk_new_open ../block/block-backend.c:457
    #12 0x56151a388bd9 in blockdev_init ../blockdev.c:612
    #13 0x56151a38ab2d in drive_new ../blockdev.c:1006
    #14 0x5615190fca41 in drive_init_func ../system/vl.c:649
    #15 0x56151aa796dd in qemu_opts_foreach ../util/qemu-option.c:1135
    #16 0x5615190fd2b6 in configure_blockdev ../system/vl.c:708
    #17 0x56151910a307 in qemu_create_early_backends ../system/vl.c:2004
    #18 0x561519113fcf in qemu_init ../system/vl.c:3685
    #19 0x56151a7e438e in main ../system/main.c:47
    #20 0x7f72d1a46249 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
    #21 0x7f72d1a46304 in __libc_start_main_impl ../csu/libc-start.c:360
    #22 0x561517e98510 in _start (/home/user/.work/qemu/build/qemu-system-aarch64+0x3b9b510)

The offset used can easily go beyond entry->name size. It's probably a
bug, but I don't have the time to dive into vfat specifics for now.

This change solves the ubsan issue, and is functionally equivalent, as
anything written past the entry->name array would not be read anyway.

Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>