[http] GET / if URI doesn't contain a path

Commit 3d9dd93 introduced a regression in HTTP: if a URI without a
path is specified (e.g. http://netboot.me), we send the empty string
as our GET request. Reintroduce an extra slash when uri->path is NULL,
to turn this into the expected GET /.

Reported-by: Kyle Kienapfel <doctor.whom@gmail.com>
Signed-off-by: Joshua Oreman <oremanj@rwcr.net>
Signed-off-by: Marty Connor <mdc@etherboot.org>

src/net/tcp/http.c

@@ -452,11 +452,12 @@ static void http_step ( struct process *process ) {
 
 		/* Send GET request */
 		if ( ( rc = xfer_printf ( &http->socket,
-					  "GET %s HTTP/1.0\r\n"
+					  "GET %s%s HTTP/1.0\r\n"
 					  "User-Agent: gPXE/" VERSION "\r\n"
 					  "%s%s%s"
 					  "Host: %s\r\n"
 					  "\r\n",
+					  http->uri->path ? "" : "/",
 					  request,
 					  ( user ?
 					    "Authorization: Basic " : "" ),