cachedb_memcached: fix NULL deref when memcached_create() returns NULL

memcached_create(NULL) can return NULL on allocation failure.  The existing
code never checks the return value, so a NULL memc pointer falls through to
memcached_server_push(NULL, ...) which dereferences the NULL pointer.

Add an explicit NULL check after memcached_create(), following the existing
error-handling pattern in the function (pkg_free + return 0).

Found during a systematic audit of cachedb backends following the
cachedb_redis NULL-deref fix in commit 8fb569c.

Author: dondetir

modules/cachedb_memcached/cachedb_memcached.c

@@ -400,6 +400,11 @@ memcached_con* memcached_new_connection(struct cachedb_id *id)
 	con->ref = 1;
 
 	con->memc = memcached_create(NULL);
+	if (!con->memc) {
+		LM_ERR("failed to create memcached handle\n");
+		pkg_free(con);
+		return 0;
+	}
 
 	memset(host_buff,0,MAX_HOSTPORT_SIZE);