Fix skipping next breakpoint unexpectedly

[iberianpig]

Description

With this PR, even if you press Enter key repeatedly before reaching the breakpoint, continue will not be executed automatically.

Issue

When you reach a breakpoint in the debugger once, proceed with 'c', and then press 'Enter' before reaching the next breakpoint, the next breakpoint is skipped. (Issue: https://github.com/ruby/debug/issues/1098)

reproduce script:

require 'debug'

def test_issue_key_strokes
  puts 'Please press "C" once and continue'
  debugger # Stop here.

  puts 'then press "Retern" once within 5 seconds.'
  sleep 5

  puts 'folloing break point will be skipped'

  debugger # Skipped! It should stop here.
end

puts 'Start'
test_issue_key_strokes
puts 'End'