Clear $stdin before showing prompt

Any input into `$stdin` before showing prompt will be appear
on the next prompt, and entering new empty line will dispatch
previous command.

This patch consumes all input to `$stdin` just before showing
the prompt.

Author: ko1

lib/debug/console.rb

@@ -97,6 +97,7 @@ def readline_setup prompt
       end
 
       def readline prompt
+        clear_stdin
         readline_setup prompt do
           Reline.readmultiline(prompt, true){ true }
         end
@@ -132,6 +133,7 @@ def readline_setup
         end
 
         def readline prompt
+          clear_stdin
           readline_setup
           Readline.readline(prompt, true)
         end
@@ -142,6 +144,7 @@ def history
 
       rescue LoadError
         def readline prompt
+          clear_stdin
           print prompt
           $stdin.gets
         end
@@ -152,6 +155,16 @@ def history
       end
     end
 
+    def clear_stdin
+      # consume all STDIN input just before prompt
+      loop do
+        case $stdin.read_nonblock(1024, exception: false)
+        when nil, :wait_readable
+          break
+        end
+      end
+    end
+
     def history_file
       path =
         if !CONFIG[:history_file].empty? && File.exist?(File.expand_path(CONFIG[:history_file]))