apparmor: turn bytes into null-terminated strings before calling strcspn

apparmor: turn bytes into null-terminated strings before calling strcspn

```
==70349==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6020000009fb at pc 0x000000433b70 bp 0x7ffcde087810 sp 0x7ffcde086fd0
READ of size 12 at 0x6020000009fb thread T0
    #0 0x433b6f in strcspn (/usr/bin/lxc-execute+0x433b6f)
    #1 0x7f720413a5cb in apparmor_process_label_get /home/runner/work/lxc/lxc/src/lxc/lsm/apparmor.c:449:8
    #2 0x7f720413bc2a in apparmor_prepare /home/runner/work/lxc/lxc/src/lxc/lsm/apparmor.c:1104:13
    #3 0x7f720409b6e9 in lxc_init /home/runner/work/lxc/lxc/src/lxc/start.c:848:8
    #4 0x7f72040a395a in __lxc_start /home/runner/work/lxc/lxc/src/lxc/start.c:2009:8
    #5 0x7f7203fc7186 in lxc_execute /home/runner/work/lxc/lxc/src/lxc/execute.c:99:9
    #6 0x7f7204000e44 in do_lxcapi_start /home/runner/work/lxc/lxc/src/lxc/lxccontainer.c:1112:9
    #7 0x7f7203ff0c07 in lxcapi_start /home/runner/work/lxc/lxc/src/lxc/lxccontainer.c:1149:8
    #8 0x4c6912 in main /home/runner/work/lxc/lxc/src/lxc/tools/lxc_execute.c:224:9
    #9 0x7f72034ac0b2 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x270b2)
    #10 0x41d93d in _start (/usr/bin/lxc-execute+0x41d93d)
+ echo ---

0x6020000009fb is located 0 bytes to the right of 11-byte region [0x6020000009f0,0x6020000009fb)
allocated by thread T0 here:
    #0 0x496399 in realloc (/usr/bin/lxc-execute+0x496399)
    #1 0x7f7203fcf85c in fd_to_buf /home/runner/work/lxc/lxc/src/lxc/file_utils.c:463:10
    #2 0x7f720413a52b in apparmor_process_label_get /home/runner/work/lxc/lxc/src/lxc/lsm/apparmor.c:442:8
    #3 0x7f720413bc2a in apparmor_prepare /home/runner/work/lxc/lxc/src/lxc/lsm/apparmor.c:1104:13
    #4 0x7f720409b6e9 in lxc_init /home/runner/work/lxc/lxc/src/lxc/start.c:848:8
    #5 0x7f72040a395a in __lxc_start /home/runner/work/lxc/lxc/src/lxc/start.c:2009:8
    #6 0x7f7203fc7186 in lxc_execute /home/runner/work/lxc/lxc/src/lxc/execute.c:99:9
    #7 0x7f7204000e44 in do_lxcapi_start /home/runner/work/lxc/lxc/src/lxc/lxccontainer.c:1112:9
    #8 0x7f7203ff0c07 in lxcapi_start /home/runner/work/lxc/lxc/src/lxc/lxccontainer.c:1149:8
    #9 0x4c6912 in main /home/runner/work/lxc/lxc/src/lxc/tools/lxc_execute.c:224:9
```

Signed-off-by: Evgeny Vereshchagin <evvers@ya.ru>