Error out on empty debug section

Error out on empty debug section

When encountering an empty debug section, we run into the following assert:
...
dwz: dwz.c:9901: read_dwarf: Assertion `data != NULL && data->d_buf != NULL' \
  failed.
...

Error out instead:
...
dwz: a.out: Found empty .debug_gdb_scripts section, not attempting dwz \
  compression
...

2019-02-14  Tom de Vries  <tdevries@suse.de>

	PR dwz/24173
	* dwz.c (read_dwarf): Change assert (data->d_buf != NULL) into an error.