The new function ensures line numbers in loclists are not beyond end of
buffer. The implementation is very inefficient, because it loads all
buffers in memory.
`eslint` and `lessc` checkers need this function.
Error sorting can't be done as a postprocess function called from
SyntasticMake(), since the final values of some relevant fields (f.i.
type) might not be known yet at that point. Solution: move sorting to
getLocListRaw(), after per-checker quiet_messages. New checker methods
getWantSort() / setWantSort() are needed.
Second problem: some checkers return screen columns mixed with byte
indices. Solution: compute screen columns as needed. Sadly, everything
about working with screen columns is fragile.
