libdwfl: remove broken coalescing logic in dwfl_report_segment

libdwfl: remove broken coalescing logic in dwfl_report_segment

dwfl_report_segment has some logic that detects when a segment is
contiguous with the previously reported segment, in which case it's
supposed to coalesce them. However, in this case, it actually returns
without updating the segment array at all. As far as I can tell, this
has always been broken. It appears that no one uses the coalescing logic
anyways, as they pass IDENT as NULL. Let's just get rid of the logic and
add a test case.

Signed-off-by: Omar Sandoval <osandov@fb.com>