telnet.c: fix invalid use of custom read function if not being set

obj_count can be 1 if the custom read function is set or the stdin
handle is a reference to a pipe. Since the pipe should be handled
using the PeekNamedPipe-check below, the custom read function should
only be used if it is actually enabled.

lib/telnet.c

@@ -1427,7 +1427,7 @@ static CURLcode telnet_do(struct connectdata *conn, bool *done)
     case WAIT_TIMEOUT:
     {
       for(;;) {
-        if(obj_count == 1) {
+        if(data->set.is_fread_set) {
           /* read from user-supplied method */
           result = (int) conn->fread_func(buf, 1, BUFSIZE - 1, conn->fread_in);
           if(result == CURL_READFUNC_ABORT) {