zebra: Fix coverity issue in dplane_srv6_encap_srcaddr_set

[cscarpitta]

Fix the following coverity issue:

*** CID 1575079:  Null pointer dereferences  (REVERSE_INULL) /zebra/zebra_dplane.c: 5950 in dplane_srv6_encap_srcaddr_set()
5944     	if (ret == AOK)
5945     		result = ZEBRA_DPLANE_REQUEST_QUEUED;
5946     	else {
5947     		atomic_fetch_add_explicit(&zdplane_info
5948     						   .dg_srv6_encap_srcaddr_set_errors,
5949     					  1, memory_order_relaxed);
     CID 1575079:  Null pointer dereferences  (REVERSE_INULL)
     Null-checking "ctx" suggests that it may be null, but it has already been dereferenced on all paths leading to the check.
5950     		if (ctx)
5951     			dplane_ctx_free(&ctx);
5952     	}
5953     	return result;
5954     }
5955

Remove the pointer check for ctx. At this point in the function it has to be non null since we deref'ed it. Additionally the alloc function that creates it cannot fail.