libdwfl: Fix assert and set in.d_size in dwfl_link_map_report.

libdwfl: Fix assert and set in.d_size in dwfl_link_map_report.

When reading the phdrs from a core file segment fails we would try to read
them from the exec. This fallback path contained a faulty assert that the
requested size of the phdrs was still setup. But the core file reading
callback might have reset the d_size value to zero on error. So explicitly
set the number of bytes we want to read again instead of asserting the size.

https://bugzilla.redhat.com/show_bug.cgi?id=1365812

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