rb_wait_for_single_fd: do not OOM or segfault with invalid FD on select()

Instead, match the poll() implementation used on Linux for now;
as the Linux poll(2) manpage describes using negative FD to
easily ignore an FD in a larger FD set while (sleeping the given
timeout).  I'm not entirely sure if matching poll() behavior
is a good idea for a single FD, but it's better than segfaulting
or NoMemoryError.

* thread.c (init_set_fd): ignore negative FD
* test/-ext-/wait_for_single_fd/test_wait_for_single_fd.rb
  (test_wait_for_invalid_fd): check values which may trigger
  segfaults or OOM

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58925 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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