DwarfAccelTable: fix obvious typo.

I have a test for that issue, but I didn't include it in the commit as it's
a 200KB file for a pretty minor issue. (The reason the file is so big is
that it needs > 1024 variables/functions to trigger and that with debug
information.

The issue/fix on the other side is totally trivial. If poeple want the test
commited, I can do that. It just didn't seem worth it to me.

llvm-svn: 231701