libsepol: do not pass NULL to memcpy

For the first iteration `mod->perm_map[sclassi]` is NULL, thus do not
use it as source of a memcpy(3), even with a size of 0.  memcpy(3) might
be annotated with the function attribute nonnull and UBSan then
complains:

    link.c:193:3: runtime error: null pointer passed as argument 2, which is declared to never be null

Signed-off-by: Christian Göttsche <cgzones@googlemail.com>

libsepol/src/link.c

@@ -190,8 +190,9 @@ static int permission_copy_callback(hashtab_key_t key, hashtab_datum_t datum,
 			ERR(state->handle, "Out of memory!");
 			return -1;
 		}
-		memcpy(newmap, mod->perm_map[sclassi],
-		       mod->perm_map_len[sclassi] * sizeof(*newmap));
+		if (mod->perm_map_len[sclassi] > 0) {
+			memcpy(newmap, mod->perm_map[sclassi], mod->perm_map_len[sclassi] * sizeof(*newmap));
+		}
 		free(mod->perm_map[sclassi]);
 		mod->perm_map[sclassi] = newmap;
 		mod->perm_map_len[sclassi] = perm->s.value;