ada: Fix New_Char_Array with empty arrays

ada: Fix New_Char_Array with empty arrays

This patch fixes an integer underflow issue on calls of the form
New_Char_Array (X) with X'Last < X'First - 2. That integer underflow
caused attempts at allocating impossibly large amount of memory in some
cases.

gcc/ada/ChangeLog:

	* libgnat/i-cstrin.adb (Position_Of_Nul): Change specification and
	adjust body accordingly.
	(New_Char_Array): Fix size of allocation.
	(To_Chars_Ptr): Adapt to Position_Of_Nul change.