Fix config validation fail when no new line from end of file

1. i3 config validation failed when the new line is missing from the end of file.
   The error was: "ERROR: Your line continuation is too long, it exceeds 4096 bytes".
   It is wrong to assume that there is always a '\n' at the end of each line in the config file.
   (Not for the last line.)
   Fix it via adding a end-of-file check.

2. See the issue #2051. (https://github.com/i3/i3/issues/2051)