Tidy objdump opb expressions

I don't think any of these can overflow, but since all of the
expressions I'm editing here are inside a while loop with condition
addr_offset < stop_offset, this change makes it more obvious that they
can't overflow.

	* objdump.c (disassemble_bytes): Calculate octet expressions
	involving both addr_offset and stop_offset by first
	subtracting addr_offset from stop_offset.