_bfd_elf_slurp_version_tables tidy

_bfd_elf_slurp_version_tables tidy

The internal verref buffer is allocated with bfd_zalloc, based on a
count given in headers.  If the headers are broken/fuzzed and contain
an enormous count the alloc can result in OOM.  If we first read the
external verrefs (into a buffer that isn't zeroed on allocation) then
the read provides a sanity check on the headers.

Also prints an error for version info that fails other sanity checks.

	* elf.c (_bfd_elf_slurp_version_tables): Delay allocation of
	internal verref buffer.  Error for zero sh_info.  Print errors.
	Check for zero vd_ndx.  Use bfd_zalloc for vd_auxptr buffer.