dwfl_offline_section_address: replace asserts with early return

dwfl_offline_section_address: replace asserts with early return

dwfl_offline_section_address asserts that the current module is ET_REL.

A possibly corrupt .gnu_debuglink can cause an abort by calling
dwfl_offline_section_address on an ET_DYN module.

Prevent this abort and similar ones by replacing
dwfl_offline_section_address initial asserts with an early return.

https://sourceware.org/bugzilla/show_bug.cgi?id=30980

Signed-off-by: Aaron Merey <amerey@redhat.com>