[Ada/varobj] number of children of null pointer to dynamic array.

[Ada/varobj] number of children of null pointer to dynamic array.

This is preparation work to avoid a regression in the Ada/varobj.
An upcoming patch is going to add support for types in DWARF
which have dynamic properties whose value is a reference to another
DIE.

Consider for instance the following declaration:

   type Variant_Type (N : Int := 0) is record
      F : String(1 .. N) := (others => 'x');
   end record;
   type Variant_Type_Access is access all Variant_Type;
   VTA : Variant_Type_Access := null;

This declares a variable "VTA" which is an access (=pointer)
to a variant record Variant_Type. This record contains two
components, the first being "N" (the discriminant), and the
second being "F", an array whose lower bound is 1, and whose
upper bound depends on the value of "N" (the discriminant).

Of interest to us, here, is that second component ("F"), and
in particular its bounds. The debugging info, and in particular
the info for the array looks like the following...

        .uleb128 0x9    # (DIE (0x91) DW_TAG_array_type)
        .long   .LASF16 # DW_AT_name: "bar__variant_type__T2b"
        .long   0xac    # DW_AT_GNAT_descriptive_type
        .long   0x2cb   # DW_AT_type
        .long   0xac    # DW_AT_sibling
        .uleb128 0xa    # (DIE (0xa2) DW_TAG_subrange_type)
        .long   0xc4    # DW_AT_type
        .long   0x87    # DW_AT_upper_bound
        .byte   0       # end of children of DIE 0x91

... where the upper bound of the array's subrange type is a reference
to "n"'s DIE (0x87):

        .uleb128 0x8    # (DIE (0x87) DW_TAG_member)
        .ascii "n\0"    # DW_AT_name
        [...]

Once the patch to handle this dynamic property gets applied,
this is what happens when creating a varobj for variable "VTA"
(whose value is null), and then trying to list its children:

    (gdb)
    -var-create vta * vta
    ^done,name="vta",numchild="2",value="0x0",
          type="bar.variant_type_access",has_more="0"
    (gdb)
    -var-list-children 1 vta
    ^done,numchild="2",
          children=[child={name="vta.n",[...]},
                    child={name="vta.f",exp="f",
                           numchild="43877616",  <<<<-----
                           value="[43877616]",   <<<<-----
                           type="array (1 .. n) of character"}],
          has_more="0"

It has an odd number of children.

In this case, we cannot really determine the number of children,
since that number depends on the value of a field in a record
for which we do not have a value. Up to now, the value we've been
displaying is zero - meaning we have an empty array.

What happens in this case, is that, because the VTA is a null pointer,
we're not able to resolve the pointer's target type, and therefore
end up asking ada_varobj_get_array_number_of_children to return
the number of elements in that array; for that, it relies blindly
on get_array_bounds, which assumes the type is no longer dynamic,
and therefore the reads the bound without seeing that it's value
is actually a reference rather than a resolved constant.

This patch prevents the issue by explicitly handling the case of
dynamic arrays, and returning zero child in that case.

gdb/ChangeLog:

        * ada-varobj.c (ada_varobj_get_array_number_of_children):
        Return zero if PARENT_VALUE is NULL and parent_type's
        range type is dynamic.

gdb/testsuite/ChangeLog:

        * gdb.ada/mi_var_array: New testcase.

Tested on x86_64-linux.