[gdb/symtab] Fix bad compile unit index complaint

[gdb/symtab] Fix bad compile unit index complaint

I noticed this code in dw2_debug_names_iterator::next:
...
        case DW_IDX_compile_unit:
          /* Don't crash on bad data.  */
          if (ull >= per_bfd->all_comp_units.size ())
            {
              complaint (_(".debug_names entry has bad CU index %s"
                           " [in module %s]"),
                         pulongest (ull),
                         objfile_name (objfile));
              continue;
            }
          per_cu = per_bfd->get_cu (ull);
          break;
...

This code used to DTRT, before we started keeping both CUs and TUs in
all_comp_units.

Fix by using "per_bfd->all_comp_units.size () - per_bfd->tu_stats.nr_tus"
instead.

It's hard to produce a test-case for this, but let's try at least to trigger
the complaint somehow.  We start out by creating an exec with .debug_types and
.debug_names:
...
$ gcc -g ~/hello.c -fdebug-types-section
$ gdb-add-index -dwarf-5 a.out
...
and verify that we don't see any complaints:
...
$ gdb -q -batch -iex "set complaints 100" ./a.out
...

We look at the CU and TU table using readelf -w and conclude that we have
nr_cus == 6 and nr_tus == 1.

Now override ull in dw2_debug_names_iterator::next for the DW_IDX_compile_unit
case to 6, and we have:
...
$ gdb -q -batch -iex "set complaints 100" ./a.out
During symbol reading: .debug_names entry has bad CU index 6 [in module a.out]
...

After this, it still crashes because this code in
dw2_debug_names_iterator::next:
...
  /* Skip if already read in.  */
  if (m_per_objfile->symtab_set_p (per_cu))
    goto again;
...
is called with per_cu == nullptr.

Fix this by skipping the entry if per_cu == nullptr.

Now revert the fix and observe that the complaint disappears, so we've
confirmed that the fix is required.

A somewhat similar issue for .gdb_index in dw2_symtab_iter_next has been filed
as PR29367.

Tested on x86_64-linux, with native and target board cc-with-debug-names.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29336