Infinite loop in get_sector_list with damaged .doc file

[rossj]

Hi there. I've come across a problematic .doc file that is causing an infinite loop in get_sector_list.

It looks like the 2nd half of this .doc file is all null, so it is definitely damaged & invalid, but it would be nice to avoid the infinite loop.

In this specific case, the loop starts off with j = 0, which results in the next j value being read from sectors[312], which is all null bytes due to the file corruption. This results in an infinite loop with j = 0.

I noticed that the chkd array is not being checked. Adding if (chkd[j]) break; at the top of the loop avoids the infinite loop and results in a later exception. Perhaps it's better to throw immediately inside the loop?

[SheetJSDev]

For the test suite, can you share an example file?

That code block was spun out of the make_sector_list function, which used the chkd variable to note if we've already built a chain that used the block in question and seen to note if we've already seen a given block when we build up a specific chain. If you'd like to submit a PR, remove the chkd references in make_sector_list and copy over the seen lines.

PS: In general, half the file being null isn't necessarily a problem (if those are treated as empty FAT sectors).