[ruby/reline] fix cursor_pos regexp match

https://github.com/ruby/reline/commit/1dd80ef188

lib/reline/ansi.rb

@@ -112,10 +112,11 @@ class Reline::ANSI
       @@input.raw do |stdin|
         @@output << "\e[6n"
         @@output.flush
-        while (c = stdin.getc) != 'R'
-          res << c if c
+        while (c = stdin.getc)
+          res << c
+          m = res.match(/\e\[(?<row>\d+);(?<column>\d+)R/)
+          break if m
         end
-        m = res.match(/\e\[(?<row>\d+);(?<column>\d+)/)
         (m.pre_match + m.post_match).chars.reverse_each do |ch|
           stdin.ungetc ch
         end