Degenerate the terminal size to [$LINES, $COLUMNS] if it is unknown

This is a workaround for ruby/irb#50

Author: mame

lib/reline/ansi.rb

@@ -50,7 +50,11 @@ def self.retrieve_keybuffer
   end
 
   def self.get_screen_size
-    @@input.winsize
+    s = @@input.winsize
+    return s if s[0] > 0 && s[1] > 0
+    s = [ENV["LINES"].to_i, ENV["COLUMNS"].to_i]
+    return s if s[0] > 0 && s[1] > 0
+    [24, 80]
   rescue Errno::ENOTTY
     [24, 80]
   end