Coverity 9656: Forward NULL

This was a false positive the callpath being walked is impossible
because the splat_kmem_cache_test_kcp_alloc() function will ensure
kcp->kcp_kcd[0] is initialized to NULL.  However, there is no harm
is making this explicit for the test case so I'm adding a line to
clearly set it to correct the analysis.

module/splat/splat-kmem.c

@@ -548,6 +548,7 @@ splat_kmem_cache_test(struct file *file, void *arg, char *name,
 		return -ENOMEM;
 	}
 
+	kcp->kcp_kcd[0] = NULL;
 	kcp->kcp_cache =
 		kmem_cache_create(SPLAT_KMEM_CACHE_NAME,
 				  kcp->kcp_size, kcp->kcp_align,