[gdb/corefiles] Fix segfault in core_target_open

On x86_64-freebsd, with test-case gdb.arch/i386-biarch-core.exp I run into a
segfault here in corelow.c:core_target_open:
...
    {
      gdb::unique_xmalloc_ptr<char> failing_command = make_unique_xstrdup
        (bfd_core_file_failing_command (current_program_space->core_bfd ()));
      if (failing_command != nullptr)
        gdb_printf (_("Core was generated by `%s'.\n"),
                    failing_command.get ());
    }
...
where bfd_core_file_failing_command returns nullptr, so the segfault happens
somewhere during "strdup (nullptr)".

There doesn't seem to be a need to make a copy of the string, so fix this by
dropping the make_unique_xstrdup.

Tested on x86_64-linux.
Tested the test-case on x86_64-freebsd.

Approved-By: Tom Tromey <tom@tromey.com>

PR corefiles/32634
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32634