libdwfl: Close ar members when they cannot be processed.

libdwfl: Close ar members when they cannot be processed.

When reporting ar members they should be closed when they cannot
be processed. A comment in offline.c said that process_file called
elf_end if it returned NULL. But this is incorrect. And other places
that call process_file do call elf_end explicitly when it returns
NULL.

Signed-off-by: Mark Wielaard <mark@klomp.org>