PR go/61303
    runtime: don't overallocate in select code
    
    If we've already allocated an fd_set, don't allocate another one.
    
    Also, don't bother to read from rdwake if it wasn't returned in select.
    
    Fixes https://gcc.gnu.org/PR61303.
    
    Reviewed-on: https://go-review.googlesource.com/17243

From-SVN: r230922