libdwfl: Fix build_id memory leak in dwfl_segment_report_module.

libdwfl: Fix build_id memory leak in dwfl_segment_report_module.

We might already have allocated memory to hold the build_id early in
consider_notes when we called consider_phdr for the program headers
we've read from the image. We would leak that memory when we don't use
it then because we return early/fail. This can be because either we
didn't find the correct bias or we skip the module because it would
conflict in address space with any already existing module of DWFL.
In both cases explicitly free the build_id memory.

Signed-off-by: Mark Wielaard <mjw@redhat.com>