BUG/MINOR: hlua: fix optional timeout argument index for AppletTCP:receive()

Baptiste reported that using the new optional timeout argument introduced
in 19e48f2 ("MINOR: hlua: add an optional timeout to AppletTCP:receive()")
the following error would occur at some point:

runtime error: file.lua:lineno: bad argument #-2 to 'receive' (number
expected, got light userdata) from [C]: in method 'receive...

In fact this is caused by exp_date being retrieved using relative index -1
instead of absolute index 3. Indeed, while using relative index is fine
most of the time when we trust the stack, when combined with yielding the
top of the stack when resuming from yielding is not necessarily the same
as when the function was first called (ie: if some data was pushed to the
stack in the yieldable function itself). As such, it is safer to use
explicit index to access exp_date variable at position 3 on the stack.

It was confirmed that doing so addresses the issue.

No backport needed unless 19e48f2 is.

src/hlua.c

@@ -5321,7 +5321,7 @@ __LJMP static int hlua_applet_tcp_recv_yield(lua_State *L, int status, lua_KCont
 	struct hlua_appctx *luactx = MAY_LJMP(hlua_checkapplet_tcp(L, 1));
 	struct stconn *sc = appctx_sc(luactx->appctx);
 	size_t len = MAY_LJMP(luaL_checkinteger(L, 2));
-	int exp_date = MAY_LJMP(luaL_checkinteger(L, -1));
+	int exp_date = MAY_LJMP(luaL_checkinteger(L, 3));
 	int ret;
 	const char *blk1;
 	size_t len1;