isisd: fix srv6_sid memory leak

isisd: fix srv6_sid memory leak

Seen with isis_srv6_topo1 topotest.

> ==178793==ERROR: LeakSanitizer: detected memory leaks
>
> Direct leak of 56 byte(s) in 1 object(s) allocated from:
>     #0 0x7f3f63cb4a57 in __interceptor_calloc ../../../../src/libsanitizer/asan/asan_malloc_linux.cpp:154
>     #1 0x7f3f6366f8dd in qcalloc lib/memory.c:105
>     #2 0x561b810c62b7 in isis_srv6_sid_alloc isisd/isis_srv6.c:243
>     #3 0x561b8111f944 in isis_zebra_srv6_sid_notify isisd/isis_zebra.c:1534
>     #4 0x7f3f637df9d7 in zclient_read lib/zclient.c:4845
>     #5 0x7f3f637779b2 in event_call lib/event.c:2011
>     #6 0x7f3f63642ff1 in frr_run lib/libfrr.c:1216
>     #7 0x561b81018bf2 in main isisd/isis_main.c:360
>     #8 0x7f3f63029d8f in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58

Fixes: 0af0f4616d ("isisd: Receive SRv6 SIDs notifications from zebra")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>