Patch: Handle relative paths in .debug_line

[Jim Blandy]

Bryce McKinlay <mckinlay@redhat.com> writes:
> Jim Blandy wrote:
> >Almost --- dwarf2_start_subfile still passes comp_dir to concat
> >without checking whether it's NULL.
> >
> Fixed in the following patch. OK?

Looks better!

You've done before-and-after runs of the test suite, right?  If there
are no regressions there, please feel free to commit. 

Are there any tests currently in the test suite that catch this bug?