ada: Extend precondition of Interfaces.C.String.Value with Length

The existing precondition guarded against exception Dereference_Error,
but not against Constraint_Error.

The RM rule B.3.1(36/3) only mentions Constraint_Error for the Value
function which returns char_array, but the one which returns String
has the same restriction, because it is equivalent to calling the
variant which returns char_array and then converted.

gcc/ada/

	* libgnat/i-cstrin.ads (Value): Extend preconditions; adapt comment for
	the package.