Fixes `InvalidIndex` readline history error
Fixes `InvalidIndex` readline history error

This would only happen when ruby compiled against libedit. History with
libedit is not fully functional but with this a least the debugger
doesn't crash.

yui-knk/byebug,deivid-rodriguez/byebug,yui-knk/byebug,deivid-rodriguez/byebug,deivid-rodriguez/byebug,yui-knk/byebug