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, 52 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.ada/mi_var_access.exp b/gdb/testsuite/gdb.ada/mi_var_access.exp
new file mode 100644
index 00000000000..5ffc0ae8772
--- /dev/null
+++ b/gdb/testsuite/gdb.ada/mi_var_access.exp
@@ -0,0 +1,52 @@
+# Copyright 2020 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib "ada.exp"
+
+if {[skip_ada_tests]} {
+    return -1
+}
+
+standard_ada_testfile mi_access
+
+load_lib mi-support.exp
+set MIFLAGS "-i=mi"
+
+if {[gdb_compile_ada "${srcfile}" "${binfile}" executable debug] != "" } {
+    return -1
+}
+
+if {[mi_gdb_start]} {
+    continue
+}
+
+mi_delete_breakpoints
+mi_gdb_reinitialize_dir $srcdir/$subdir
+mi_gdb_load ${binfile}
+
+if ![mi_run_to_main] then {
+    fail "cannot run to main, testcase aborted"
+    return 0
+}
+
+set bp_location [gdb_get_line_number "STOP" ${testdir}/mi_access.adb]
+mi_continue_to_line \
+    "mi_access.adb:$bp_location" \
+    "stop at start of mi_access"
+
+# The bug was that creating a varobj for A_String_Access would crash.
+mi_gdb_test "-var-create A_String_Access * A_String_Access" \
+    "\\^done,name=\"A_String_Access\",numchild=\"1\",.*" \
+    "Create varobj"