[Ada] check for null picture string

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk

A null picture string caused Constraint_Error to be raised
instead of Picture_Error.

The following program

with Ada.Calendar; use Ada.Calendar;
with GNAT.Calendar.Time_IO; use GNAT.Calendar.Time_IO;
procedure t is
    S : String := Image (Clock, "");
begin
    null;
end;

should generate as output:

raised GNAT.CALENDAR.TIME_IO.PICTURE_ERROR : null picture string

As is seen from this example, message text has been added to the
raise statements.

2007-06-06  Robert Dewar  <dewar@adacore.com>

	* g-catiio.ads, g-catiio.adb (Image): Check for null picture string

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