objcopy buffer overflow

"tocopy" in this code was an int, which when the size to be copied was
larger than MAXINT could result in tocopy being negative.  A negative
value of course is less than BUFSIZE, but when converted to
bfd_size_type is extremely large.

	PR 995
	* objcopy.c (copy_unknown_object): Correct calculation of "tocopy".
	Use better variable types.