fix line getting handler of line_ary
update filename before appending file debug info
remove unnecessary scope filename update
check file debug info is appended correctly
Merge pull request #1500 from take-cheeze/debug_info_file_updating

Fix insufficient preparation to call mrb_debug_info_append_file.
Fix checking return value of fread()
Merge pull request #1501 from iij/pr_fix_read_debug_section

Fix checking return value of fread()