Prohibit sundown-added LFs from source maps

This commit addresses the "+1 LF" issue Sundown is responsible for – If an input markdown buffer does not contain a trailing LF sundown adds an artificial LF in order for parsing to work. This however affects the sundown source maps as well. This is a Sundown design flaw and should be eventually addressed there. Until then we will reduce to add or truncate any source map that exceeds actual buffer size.