Fix receiveheaders function

[AMD-NICK]

It seems that error checking is needed here, otherwise in theory there will be an error in the while loop if line becomes false

[AMD-NICK]

Lua Test code:

local http = require("socket.http")
local _, err = http.request("http://dev:port", "")
print("err", err)

nodejs

const net = require('net');

const server = net.createServer((socket) => {
    socket.write("HTTP/1.1 200 OK\r\n");
    socket.write("Content-Type: text/plain\r\n");
    socket.write("X-Custom-Header: key=val;\r\n");
	socket.write("    foo=bar;\r\n")
	socket.end(); // <<----------------------------------
	socket.write("    bar=baz;\r\n")
    socket.write("\r\n");
    socket.write("Hello, World!\r\n");
    
    socket.end();
});

const PORT = 8000;
server.listen(PORT, () => {
    console.log(`Server listening on port ${PORT}`);
});

Without fix:

......./share/lua/5.1/socket/http.lua:50: bad argument #1 to 'find' (string expected, got nil)

with:

err closed

[alerque]

Thanks for taking the time to track this down and contribute a fix.