In some cases involving null slices, the Slice subprograms (both the
  function and the procedure) in each of the three Generic_Bounded_Length
  generic packages (for String, Wide_String, and Wide_Wide_String)
  could raise Constraint_Error in cases where this is incorrect.

Tested on x86_64-pc-linux-gnu, committed on trunk

gcc/ada/

	* libgnat/a-strsup.adb, libgnat/a-stwisu.adb, libgnat/a-stzsup.adb
	(Super_Slice function and procedure): fix slice length computation.