Fix MI crash with Ada string access

I happened to notice that using -var-create at a certain spot in an
Ada program caused a crash.  This happens because
ada_get_decoded_value can return NULL -- in particular, deeper in the
code it can hit this return in ada_type_of_array:

      descriptor = desc_bounds (arr);
      if (value_as_long (descriptor) == 0)
        return NULL;

This patch avoids the crash by handling this NULL return.

gdb/ChangeLog
2020-08-17  Tom Tromey  <tromey@adacore.com>

	* ada-varobj.c (ada_varobj_decode_var): Handle case where
	ada_get_decoded_value returns NULL.

gdb/testsuite/ChangeLog
2020-08-17  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/mi_var_access.exp: New file.
	* gdb.ada/mi_var_access/mi_access.adb: New file.
	* gdb.ada/mi_var_access/pck.adb: New file.
	* gdb.ada/mi_var_access/pck.ads: New file.

1 files changed, 4 insertions, 4 deletions

diff --git a/gdb/ada-varobj.c b/gdb/ada-varobj.c
index f67fe5002a3..a72653af37a 100644
--- a/gdb/ada-varobj.c
+++ b/gdb/ada-varobj.c
@@ -63,10 +63,10 @@ static void
 ada_varobj_decode_var (struct value **value_ptr, struct type **type_ptr)
 {
   if (*value_ptr)
-    {
-      *value_ptr = ada_get_decoded_value (*value_ptr);
-      *type_ptr = ada_check_typedef (value_type (*value_ptr));
-    }
+    *value_ptr = ada_get_decoded_value (*value_ptr);
+
+  if (*value_ptr != nullptr)
+    *type_ptr = ada_check_typedef (value_type (*value_ptr));
   else
     *type_ptr = ada_get_decoded_type (*type_ptr);
 }