PR32032 dwp segfaults on hello world binary

Fixing the segfault is easy with this bandaid, but further work is
needed to teach dwp about DW_AT_dwo_name and dwo id in the cu header.
At the moment dwp only handles DW_AT_GNU_dwo_name and DW_AT_GNU_dwo_id.

	PR 32032
	* dwp.cc (Dwp_output_file::finalize): Return immediately on
	no output file.