Fix empty port assertion error in 'Join Game' menu

Pressing "Register" when the port field was empty led to an assertion error inside `create_register_dialog`, it was simply checking the port was a number.

builtin/mainmenu/tab_online.lua

@@ -346,12 +346,14 @@ local function main_button_handler(tabview, fields, name, tabdata)
 		return true
 	end
 
-	if (fields.btn_mp_login or fields.key_enter)
-			and fields.te_address ~= "" and fields.te_port then
+	local host_filled = (fields.te_address ~= "") and fields.te_port:match("^%s*[1-9][0-9]*%s*$")
+	local te_port_number = tonumber(fields.te_port)
+
+	if (fields.btn_mp_login or fields.key_enter) and host_filled then
 		gamedata.playername = fields.te_name
 		gamedata.password   = fields.te_pwd
 		gamedata.address    = fields.te_address
-		gamedata.port       = tonumber(fields.te_port)
+		gamedata.port       = te_port_number
 
 		local enable_split_login_register = core.settings:get_bool("enable_split_login_register")
 		gamedata.allow_login_or_register = enable_split_login_register and "login" or "any"
@@ -391,10 +393,10 @@ local function main_button_handler(tabview, fields, name, tabdata)
 		return true
 	end
 
-	if fields.btn_mp_register and fields.te_address ~= "" and fields.te_port then
+	if fields.btn_mp_register and host_filled then
 		local idx = core.get_table_index("servers")
 		local server = idx and tabdata.lookup[idx]
-		if server and (server.address ~= fields.te_address or server.port ~= tonumber(fields.te_port)) then
+		if server and (server.address ~= fields.te_address or server.port ~= te_port_number) then
 			server = nil
 		end
 
@@ -403,7 +405,7 @@ local function main_button_handler(tabview, fields, name, tabdata)
 			return true
 		end
 
-		local dlg = create_register_dialog(fields.te_address, tonumber(fields.te_port), server)
+		local dlg = create_register_dialog(fields.te_address, te_port_number, server)
 		dlg:set_parent(tabview)
 		tabview:hide()
 		dlg:show()