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