gdb/testsuite: Add test for case where gdb_demangle returns NULL

This adds a test for the commit:

  commit 4f180d5396
  Date:   Fri Feb 21 08:19:21 2020 -0700

      Check for null result from gdb_demangle

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/cpp-linkage-name.c: New file.
	* gdb.dwarf2/cpp-linkage-name.exp: New file.

gdb/testsuite/ChangeLog

@@ -1,3 +1,8 @@
+2020-02-21  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.dwarf2/cpp-linkage-name.c: New file.
+	* gdb.dwarf2/cpp-linkage-name.exp: New file.
+
 2020-02-21  Shahab Vahedi  <shahab@synopsys.com>
 
 	* lib/gdb.exp (gdb_wrapper_init): Reset

gdb/testsuite/gdb.dwarf2/cpp-linkage-name.c

@@ -0,0 +1,29 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   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/>.  */
+
+volatile struct
+{
+  int x;
+  int y;
+} global_var;
+
+int
+main (void)
+{
+  asm ("main_label: .globl main_label");
+  return global_var.x + global_var.y;
+}

gdb/testsuite/gdb.dwarf2/cpp-linkage-name.exp

@@ -0,0 +1,96 @@
+# 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/>.
+
+# Some compilers give anonymous structures a linkage name, and that
+# linkage name doesn't demangle (within GDB calling gdb_demangle
+# return NULL).  At one point this caused GDB to crash due to
+# dereferencing a NULL pointer.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile cpp-linkage-name.c cpp-linkage-name-debug.S
+
+# Set up the DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+
+    lassign [function_range main ${srcdir}/${subdir}/${srcfile}] \
+	main_start main_length
+
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_language @DW_LANG_C_plus_plus}
+	    {DW_AT_name     ada-linkage-name.c}
+	    {DW_AT_comp_dir /tmp}
+
+	} {
+	    declare_labels a_l b_l
+
+	    a_l: DW_TAG_base_type {
+		{DW_AT_byte_size 4 DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_name      int}
+	    }
+
+	    # To expose the bug that existed at one point this
+	    # structure must have a linkage name, but no name, and the
+	    # linkage name is something that doesn't demangle.
+	    b_l: DW_TAG_structure_type {
+		{DW_AT_byte_size 8 DW_FORM_sdata}
+		{DW_AT_encoding  @DW_ATE_signed}
+		{DW_AT_linkage_name <anon>}
+	    } {
+		member {
+		    {name x}
+		    {type :$a_l}
+		    {data_member_location 0 data1}
+		}
+		member {
+		    {name y}
+		    {type :$a_l}
+		    {data_member_location 0 data1}
+		}
+	    }
+	    DW_TAG_subprogram {
+		{name "main"}
+		{low_pc $main_start addr}
+		{high_pc "$main_start + $main_length" addr}
+		{type :$a_l}
+	    }
+	    DW_TAG_variable {
+		{type :$b_l}
+		{external 1 flag}
+		{DW_AT_name global_var}
+		{DW_AT_location {
+		    DW_OP_addr [gdb_target_symbol global_var]
+		} SPECIAL_expr}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+gdb_assert [runto_main] "run to main"
+gdb_test "p global_var" " = {x = 0, y = 0}"