Ensure the passed jobserver FDs are valid

If invalid, clear `-j` option from `$MAKEFLAGS`.

Author: nobu

spec/ruby/optional/capi/spec_helper.rb

@@ -127,8 +127,12 @@ def setup_make
   opts = {}
   if /(?:\A|\s)--jobserver-(?:auth|fds)=(\d+),(\d+)/ =~ make_flags
     [$1, $2].each do |fd|
-      fd = fd.to_i(10)
+      fd = IO.for_fd(fd.to_i(10), autoclose: false)
       opts[fd] = fd
+    rescue
+      make_flags.sub!(/\A-\S*\Kj\d+/, '')
+      opts.clear
+      break
     end
   end