Fix stepping in rtld without debug symbol

Fix stepping in rtld without debug symbol

Commit be6276e0aed "Allow debugging of runtime loader / dynamic linker"
introduced a small regression when stepping into the runtime loader /
dynamic linker from function we do not have debug information for.  This
is reported in PR/29747.

This can be shown by the following example (given by Simon Marchi in
buzilla bug report):

    $ cat test.c
    #include <stdio.h>

    int main()
    {
      printf("Hi\n");
      return 0;
    }
    $ gcc test.c -O0 -o test
    $ ./gdb -q -nx --data-directory=data-directory test -ex start -ex s
    Reading symbols from test...
    (No debugging symbols found in test)
    Temporary breakpoint 1 at 0x1151
    Starting program: .../binutils-gdb/gdb/test
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".

    Temporary breakpoint 1, 0x0000555555555151 in main ()
    Single stepping until exit from function main,
    which has no line number information.
    /home/smarchi/src/binutils-gdb/gdb/infrun.c:6960:64: runtime error: member call on null pointer of type 'struct symbol'

    The crash happens here:

    #0  __sanitizer::Die () at ../../../../src/libsanitizer/sanitizer_common/sanitizer_termination.cpp:50
    #1  0x00007ffff5dd7128 in __ubsan::__ubsan_handle_type_mismatch_v1_abort (Data=<optimized out>, Pointer=<optimized out>) at ../../../../src/libsanitizer/ubsan/ubsan_handlers.cpp:148
    #2  0x000055556183e1a7 in process_event_stop_test (ecs=0x7fffffffccd0) at .../binutils-gdb/gdb/infrun.c:6960
    #3  0x0000555561838ea4 in handle_signal_stop (ecs=0x7fffffffccd0) at .../binutils-gdb/gdb/infrun.c:6615
    #4  0x000055556182f77b in handle_inferior_event (ecs=0x7fffffffccd0) at .../binutils-gdb/gdb/infrun.c:5866

When evaluating:

    6956   if (execution_direction != EXEC_REVERSE
    6957       && ecs->event_thread->control.step_over_calls == STEP_OVER_UNDEBUGGABLE
    6958       && in_solib_dynsym_resolve_code (ecs->event_thread->stop_pc ())
    6959       && !in_solib_dynsym_resolve_code (
    6961          ecs->event_thread->control.step_start_function->value_block ()
    6962              ->entry_pc ()))

we dereference, ecs->event_thread->control.step_start_function which is
nullptr.

This patch changes this condition so it evaluates to true if
ecs->event_thread->control.step_start_function is nullptr since this
matches the behaviour before be6276e0aed.

Tested on ubuntu-22.04 x86_64.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29747
Reviewed-By: Bruno Larsen <blarsen@redhat.com>
Approved-By: Kevin Buettner <kevinb@redhat.com>