PR27291, integer overflow in bfd_get_section_contents

PR27291, integer overflow in bfd_get_section_contents

Makes the code a little more elegant too.  Note that the unsigned
overflow reported here is well defined so this patch doesn't fix any
real problem.

	PR 27291
	* section.c (bfd_get_section_contents): Avoid possible overflow
	when range checking offset and count.
	(bfd_set_section_contents): Likewise.