zebra: Free Zebra client resources

zebra: Free Zebra client resources

Memory leaks started flowing:

```
AddressSanitizer Topotests Part 0:  15 KB -> 283 KB
AddressSanitizer Topotests Part 1:  1 KB -> 495 KB
AddressSanitizer Topotests Part 2:  13 KB -> 478 KB
AddressSanitizer Topotests Part 3:  39 KB -> 213 KB
AddressSanitizer Topotests Part 4:  30 KB -> 836 KB
AddressSanitizer Topotests Part 5:  0 bytes -> 356 KB
AddressSanitizer Topotests Part 6:  86 KB -> 783 KB
AddressSanitizer Topotests Part 7:  0 bytes -> 354 KB
AddressSanitizer Topotests Part 8:  0 bytes -> 62 KB
AddressSanitizer Topotests Part 9:  408 KB -> 518 KB
```

```
Direct leak of 3584 byte(s) in 1 object(s) allocated from:
    #0 0x7f1957b02d28 in __interceptor_calloc (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xded28)
    #1 0x559895c55df0 in qcalloc lib/memory.c:105
    #2 0x559895bc1cdf in zserv_client_create zebra/zserv.c:743
    #3 0x559895bc1cdf in zserv_accept zebra/zserv.c:880
    #4 0x559895cf3438 in event_call lib/event.c:1995
    #5 0x559895c3901c in frr_run lib/libfrr.c:1213
    #6 0x559895a698f1 in main zebra/main.c:472
    #7 0x7f195635ec86 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21c86)
```

Fixes b20acd0 ("bgpd: Use synchronous way to get labels from Zebra")

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>