lfs - tail.lua is not erroring on an invalid checkpoint about lua_sandbox_extensions

[trink]

A value beyond the EOF will not cause a seek error so I added a specific check. Otherwise we have this bug; it will simply seek there and any read attempt will return EOF until the file actually grows to that size.

As for the root cause I can think of a couple of reasons.

• Hindsight was not running when a log file was rolled (leaving an old large checkpoint in place). If that was the case this is also the fix (the data from the time it wasn't running to the roll point would not be loaded, for this plugin that is a works as designed)
• read_until_eof improperly updated the checkpoint (there are two flavors, for the mail log you are reading I assume you are not using the delimiter version, so the checkpoint is a running count of bytes read. The delimiter version is a bit more complex but I am not seeing any potential issues after a quick code review of each.

@giganteous