[Ada] print null array pointer as `0x0' rather than `(null)'

Trying to print a variable defined as an access to an unconstrained
array:

       type String_Access is access String;
       S1 : String_Access;

If that variable is null, then GDB prints its value in an odd way:

    (gdb) print S1
    $1 = (string_bug.string_access) (null)
                                    ^^^^^^

This patch changes the debugger behavior to print the pointer using
the same output we'd use for any null pointer:

    (gdb) print S1
    $1 = (string_bug.string_access) 0x0

It also adds an assert, helping us verify an assumption.

gdb/ChangeLog:

        * ada-valprint.c (ada_val_print_1): Print null array pointers as
        `0x0' rather than `(null)'.  Add assertion.

gdb/testsuite/ChangeLog:

        * gdb.ada/arrayptr/foo.adb: Add new local variable Null_String.
        * gdb.ada/arrayptr.exp: Add test printing that new variable.

1 files changed, 2 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.ada/arrayptr/foo.adb b/gdb/testsuite/gdb.ada/arrayptr/foo.adb
index 8cedd87..c11ccfb 100644
--- a/gdb/testsuite/gdb.ada/arrayptr/foo.adb
+++ b/gdb/testsuite/gdb.ada/arrayptr/foo.adb
@@ -19,6 +19,8 @@ procedure Foo is
    type String_Access is access String;
 
    String_P : String_Access := new String'("Hello");
+   Null_String : String_Access := null;
 begin
    Do_Nothing (String_P'Address);  -- STOP
+   Do_Nothing (Null_String'Address);
 end Foo;