objdump segfault after symbol table error

This memcpy segfaults if symcount is -1 (=> syms is NULL).
      memcpy (sorted_syms, symcount ? syms : dynsyms,
	      sorted_symcount * sizeof (asymbol *));

	* objdump.c (slurp_symtab): Don't leave symcount as -1 after
	an error.
	(slurp_dynamic_symtab): Likewise for dynsymcount.