PR28401, invalid section name lookup

The PR28401 testcase has a section named "", ie. an empty string.
This results in some silly behaviour in load_debug_section, and
dump_dwarf_section.  Fix that.  Note that this patch doesn't correct
the main complaint in PR28401, "failed to allocate", since malloc
failures on sections having huge bogus sizes are to be expected.  We
can't safely catch all such cases by comparing with file size, for
example, where sections contain compressed data.

	PR 28401
	* objdump.c (load_debug_section): Don't attempt to retrieve
	empty name sections.
	(dump_dwarf_section): Likewise.