lib: Fix compilation warning

lib: Fix compilation warning

Don't store unnecessary `len` returned by snprintf and snprintfrr function.

Fixes the following compilation warnings:

```
error	15-Aug-2024 13:54:10	In file included from lib/bfd.c:16:
error	15-Aug-2024 13:54:10	In file included from ./lib/zclient.h:22:
error	15-Aug-2024 13:54:10	In file included from ./lib/nexthop.h:14:
error	15-Aug-2024 13:54:10	./lib/srv6.h:338:3: warning: Value stored to 'len' is never read [deadcode.DeadStores]
error	15-Aug-2024 13:54:10	                len += snprintf(str + len, size - len, " USP");
error	15-Aug-2024 13:54:10	                ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error	15-Aug-2024 13:54:10	./lib/srv6.h:343:3: warning: Value stored to 'len' is never read [deadcode.DeadStores]
error	15-Aug-2024 13:54:10	                len += snprintfrr(str + len, size - len, " nh6 %pI6", &ctx->nh6);
error	15-Aug-2024 13:54:10	                ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error	15-Aug-2024 13:54:10	./lib/srv6.h:347:3: warning: Value stored to 'len' is never read [deadcode.DeadStores]
error	15-Aug-2024 13:54:10	                len += snprintfrr(str + len, size - len, " nh4 %pI4", &ctx->nh4);
error	15-Aug-2024 13:54:10	                ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error	15-Aug-2024 13:54:10	./lib/srv6.h:354:3: warning: Value stored to 'len' is never read [deadcode.DeadStores]
error	15-Aug-2024 13:54:10	                len += snprintf(str + len, size - len, " vrf_id %u",
error	15-Aug-2024 13:54:10	                ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error	15-Aug-2024 13:54:10	./lib/srv6.h:367:3: warning: Value stored to 'len' is never read [deadcode.DeadStores]
error	15-Aug-2024 13:54:10	                len += snprintf(str + len, size - len, " unknown(%s)", __func__);
error	15-Aug-2024 13:54:10	                ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Signed-off-by: Carmine Scarpitta <cscarpit@cisco.com>