NS32K arg_bufs uninitialised

NS32K arg_bufs uninitialised

git commit d1e304bc27 was aimed at stopping uninitialised memory
access to the index_offset array.  Unfortunately that patch resulted
in a different array being uninitialised for all instructions with
more than two arguments.

	* ns32k-dis.c (print_insn_arg): Update comment.
	(print_insn_ns32k): Reduce size of index_offset array, and
	initialize, passing -1 to print_insn_arg for args that are not
	an index.  Don't exit arg loop early.  Abort on bad arg number.