SET GLOBAL proxy_protocol_networks=(empty) can trigger assertion failure on next TCP connection

Server version: 5.5.5-10.11.17-MariaDB-debug-log Source distribution

Steps:
1. Start MariaDB debug build normally.
2. Connect to the server using a first client connection.
3. Execute:

SET GLOBAL proxy_protocol_networks=(empty);

SHOW GLOBAL VARIABLES LIKE 'proxy_protocol_networks';

4. Open a *new TCP connection* to the same server. For example:

mysql -h127.0.0.1 -P13307 -uroot -p -e "SELECT 1"

5. Observe that the server aborts during connection handling, before the query is actually executed. ERROR 2013 (HY000): Lost connection to MySQL server at 'reading initial communication packet', system error: 2