string.rb: upto to break when the string length is longer than end.

mrbgems/mruby-string-ext/mrblib/string.rb

@@ -457,6 +457,8 @@ class String
       break if exclusive and n == 0
       yield bs
       break if n == 0
+      bsiz = bs.size
+      break if bsiz > max.size || bsiz == 0
       bs = bs.succ
     end
     self