Use integer port argument with mysql_real_connect

The port argument is not a string, but a plain integer.  In the past,
compilers ignore such type errors and produced a warning only, but
this is changing, so this change also addresses a potential build
failure.

db/drivers/mysql/db.c

@@ -54,7 +54,9 @@ int db__driver_open_database(dbHandle *handle)
 
         connection = mysql_init(NULL);
         res = mysql_real_connect(connection, host, user, password,
-                                 connpar.dbname, port, NULL, 0);
+                                 connpar.dbname,
+                                 port != NULL ? atoi(port) : 0,
+                                 NULL, 0);
 
         if (res == NULL) {
             db_d_append_error("%s\n%s", _("Connection failed."),