cache: set the cache_table to NULL if empty

Instead of realloc'ing to a size of 0, set the cache_table to NULL when
removing the last element from it.

libpkgconf/cache.c

@@ -186,6 +186,9 @@ pkgconf_cache_remove(pkgconf_client_t *client, pkgconf_pkg_t *pkg)
 	}
 
 	client->cache_count--;
+	if (client->cache_count == 0)
+		client->cache_table = NULL;
+	else
 		client->cache_table = pkgconf_reallocarray(client->cache_table,
 			client->cache_count, sizeof(void *));
 }