Fix error of quitting in the middle of gtags generating (issue 319).

[iLeeWell]

Fix error of quitting in the middle of gtags generating. #319

[smemsh]

@iLeeWell if I'm reading this correctly, it removes all zero-byte files in the directory where the db is located:

let files = split(globpath(l:db_path, '*'), '\n')
for file in files
    let file_size = getfsize(file)
    if file_size == 0
        try | call delete(file) | endtry
    endif
endfor

is this not dangerous for people that are not using g:gutentags_cache_dir ?