Initialize @rbuf_empty in BufferedIO

[etiennebarrie]

#14 is causing a warning under Ruby 2.7 when the instance variable is first used without having been initialized first.

I know this warning has been removed since Ruby 3.0, but with object shapes coming up it might be good anyway to make sure all instances share all the same shape transitions that way none of the instance variable reads will get cache misses.

[hsbt]

Thanks for your report. Can you show the warnings with Ruby 2.7? I will merge this after it.

[etiennebarrie]

Of course. The test case causes the warning under Ruby 2.7 without the added line in the initializer. You can also see that with the released version like this:

echo | ruby -v \
  -e 'gem "net-protocol", "0.2.0"' \
  -e 'require "net/protocol"' \
  -e 'puts Net::Protocol::VERSION' \
  -e 'Net::BufferedIO.new(STDIN).readline'

ruby 2.7.7p221 (2022-11-24 revision 168ec2b1e5) [arm64-darwin21]
0.2.0
/Users/etienne/.gem/ruby/2.7.7/gems/net-protocol-0.2.0/lib/net/protocol.rb:216: warning: instance variable @rbuf_empty not initialized

[hsbt]

Thanks a lot. This fix seems reasonable.