gdb/dwarf2/read.c: Handle empty CU name

I recently came across a case where a compiler would emit a CU with an
empty name.  In such case, the attribute object constructed by GDB will
return nullptr when as_string is called.  One place is not checking for
this possibility.  As a result, loading such binary results in a GDB
crash:

    $ gdb -q a.out
    Reading symbols from a.out...

    Fatal signal: Segmentation fault
    ----- Backtrace -----
    [...]
    0x742f4dd8afab __strcmp_avx2
            ../sysdeps/x86_64/multiarch/strcmp-avx2.S:283
    0x58593704a0bc prepare_one_comp_unit
            ../../gdb/dwarf2/read.c:21842
    0x585937053fd9 process_psymtab_comp_unit
            ../../gdb/dwarf2/read.c:4633
    0x585937053fd9 _ZN23cooked_index_debug_info11process_cusEmN9__gnu_cxx17__normal_iteratorIPSt10unique_ptrI18dwarf2_per_cu_data26dwarf2_per_cu_data_deleterESt6vectorIS5_SaIS5_EEEESA_
            ../../gdb/dwarf2/read.c:4943
    [...]
    ---------------------
    A fatal error internal to GDB has been detected, further
    debugging is not possible.  GDB will now terminate.

    This is a bug, please report it.  For instructions, see:
    <https://www.gnu.org/software/gdb/bugs/>.

    Segmentation fault (core dumped)

This seems to be a regression introduced by the following commit:

    commit 00105aa1c4
    Date:   Tue Sep 24 10:24:22 2024 +0200

        [gdb/symtab] Don't expand non-Ada CUs for info exceptions

This patch fixes this issue by checking if attr->as_string returns
nullptr.

Change-Id: I78fe7a090f0bd1045b8cb2f8d088a8d6cf57fe1c
Approved-By: Andrew Burgess <aburgess@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>

gdb/dwarf2/read.c

@@ -21837,6 +21837,7 @@ prepare_one_comp_unit (struct dwarf2_cu *cu, struct die_info *comp_unit_die,
   attr = dwarf2_attr (comp_unit_die, DW_AT_name, cu);
   if (attr != nullptr
       && cu->producer != nullptr
+      && attr->as_string () != nullptr
       && strcmp (attr->as_string (), "<artificial>") == 0
       && producer_is_gcc (cu->producer, nullptr, nullptr))
     cu->per_cu->lto_artificial = true;

gdb/testsuite/gdb.dwarf2/cu-empty-name.c

@@ -0,0 +1,22 @@
+/* Copyright (C) 2024 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.  */
+
+int
+main ()
+{
+  return 0;
+}

gdb/testsuite/gdb.dwarf2/cu-empty-name.exp

@@ -0,0 +1,48 @@
+# Copyright 2024 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/>.
+
+# Check that GDB can load a CU with an empty name properly.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+require dwarf2_support
+
+standard_testfile .c .S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    cu {} {
+	DW_TAG_compile_unit {
+	    {DW_AT_producer "Tsetsuite"}
+	    {DW_AT_language @DW_LANG_C}
+	    {DW_AT_name     ""}
+	    {DW_AT_comp_dir /tmp}
+	} {
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+if { ![runto_main] } {
+    return -1
+}
+
+gdb_continue_to_end