Pointer comparisons after adding an offset just don't work to catch
overflow when the offset is a larger type than the pointer.

	PR 25018
	* dwarf.c (get_type_signedness): Delete ineffective pointer
	comparison check.  Properly range check uvalue offset on
	recursive call.
	(read_and_display_attr_value): Range check uvalue offset before
	calling get_type_signedness.