Internal error trying to print uninitialized string.

Trying to print the value of a string whose size is not known at
compile-time before it gets assigned a value can lead to the following
internal error:

    (gdb) p my_str
    $1 =
    /[...]/utils.c:1089: internal-error: virtual memory exhausted.

What happens is that my_str is described as a reference to an array
type whose bounds are dynamic. During the read of that variable's
value (in default_read_var_value), we end up resolving dynamic types
which, for reference types, makes us also resolve the target of that
reference type. This means we resolve our variable to a reference
to an array whose bounds are undefined, and unfortunately very far
appart.

So, when we pass that value to ada-valprint, and in particular to
da_val_print_ref, we eventually try to allocate too large of a buffer
corresponding to the (bogus) size of our array, hence the internal
error.

This patch fixes the problem by adding a size_check before trying
to print the dereferenced value. To perform this check, a function
that was previously specific to ada-lang.c (check_size) gets
exported, and renamed to something less prone to name collisions
(ada_ensure_varsize_limit).

gdb/ChangeLog:

        * ada-lang.h (ada_ensure_varsize_limit): Declare.
        * ada-lang.c (check_size): Remove advance declaration.
        (ada_ensure_varsize_limit): Renames check_size.
        Replace calls to check_size by calls to ada_ensure_varsize_limit
        throughout.
        * ada-valprint.c (ada_val_print_ref): Add call to
        ada_ensure_varsize_limit.  Add comment explaining why.

gdb/testsuite/ChangeLog:

        * gdb.ada/str_uninit: New testcase.

gdb/ada-lang.h

@@ -162,6 +162,8 @@ struct ada_task_info
 
 extern void *grow_vect (void *, size_t *, size_t, int);
 
+extern void ada_ensure_varsize_limit (const struct type *type);
+
 extern int ada_get_field_index (const struct type *type,
                                 const char *field_name,
                                 int maybe_missing);