objdump: don't add an extra entry to syms array

Space for a NULL is there in every backend bfd_get_symtab_upper_bound
or bfd_get_dynamic_symtab_upper_bound when the symbol count is non-zero,
and placed as a terminator by bfd_canonicalize_symtab.  Many backends
even return a single NULL entry array for zero symbol count, and while
there are a few that return a NULL array for no symbols, that case is
handled fine in objdump.  So don't have objdump add yet another NULL
entry.

	* objdump.c (slurp_symtab): Don't add an extra entry for NULL
	to the symbol array.
	(slurp_dynamic_symtab): Likewise.
	(dump_bfd): Formatting.  Copy terminating NULL from extra_syms.