objcopy of mach-o indirect symbols

Anti-fuzzer measure.  I'm not sure what the correct fix is for
objcopy.  Probably the BFD_MACH_O_S_NON_LAZY_SYMBOL_POINTERS,
BFD_MACH_O_S_LAZY_SYMBOL_POINTERS and BFD_MACH_O_S_SYMBOL_STUBS
contents should be read.

	* mach-o.c (bfd_mach_o_section_get_nbr_indirect): Omit sections
	with NULL sec->indirect_syms.