zebra: Fix compilation warning

zebra: Fix compilation warning

Make sure the variable `srv6` is not NULL before dereferencing it.

Fixes the following compilation warnings:

```
error	15-Aug-2024 14:20:01	zebra/zebra_srv6_vty.c:974:8: warning: Access to field 'locators' results in a dereference of a null pointer (loaded from variable 'srv6') [core.NullDereference]
error	15-Aug-2024 14:20:01	                for (ALL_LIST_ELEMENTS_RO(srv6->locators, node, locator)) {
error	15-Aug-2024 14:20:01	                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
error	15-Aug-2024 14:20:01	./lib/linklist.h:345:11: note: expanded from macro 'ALL_LIST_ELEMENTS_RO'
error	15-Aug-2024 14:20:01	        (node) = listhead(list), ((data) = NULL);                              \
error	15-Aug-2024 14:20:01	                 ^~~~~~~~~~~~~~
error	15-Aug-2024 14:20:01	./lib/linklist.h:63:22: note: expanded from macro 'listhead'
error	15-Aug-2024 14:20:01	#define listhead(X) ((X) ? ((X)->head) : NULL)
error	15-Aug-2024 14:20:01	                     ^
```

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