Make bfd_mach_o_archive_p more robust

[Nick Clifton]

Hi Andreas,

> 2009-01-27  Andreas Schwab  <schwab@suse.de>
> 
> 	* mach-o.c (bfd_mach_o_archive_p): Restrict the number of
> 	architectures in the archive.

Approved - please apply.

Cheers
   Nick