llvm-cov: Fix handling of line zero appearing in a line table

Reading line tables in llvm-cov was pretty broken, but would happen to
work as long as no line in the table was 0. It's not clear to me
whether a line of zero *should* show up in these tables, but deciding
to read a string in the middle of the line table is certainly the
wrong thing to do if it does.

I've also added some comments, as trying to figure out what this block
of code was doing was fairly unpleasant.

llvm-svn: 207866