readelf: Skip trying to uncompress sections without a name

readelf: Skip trying to uncompress sections without a name

When combining eu-readelf -z with -x or -p to dump the data or strings
in an (corrupted ELF) unnamed numbered section eu-readelf could crash
trying to check whether the section name starts with .zdebug. Fix this
by skipping sections without a name.

   * src/readelf.c (dump_data_section): Don't try to gnu decompress a
   section without a name.
   (print_string_section): Likewise.

https://sourceware.org/bugzilla/show_bug.cgi?id=32656

Signed-off-by: Mark Wielaard <mark@klomp.org>