[llvm-symbolizer] Fix section index at the end of a section

This is very minor issue. The returned section index is only used by
DWARFDebugLine as an llvm::upper_bound input and the use case shouldn't
cause any behavioral change.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@358814 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed