[gdb] Fix assert in handle_jit_event

[gdb] Fix assert in handle_jit_event

With the cc-with-tweaks.sh patch submitted here (
https://sourceware.org/pipermail/gdb-patches/2022-October/192586.html ) we run
with:
...
$ export STRIP_ARGS_STRIP_DEBUG=--strip-all
$ make check RUNTESTFLAGS="gdb.base/jit-reader.exp \
    --target_board cc-with-gnu-debuglink"
...
into the following assert:
...
(gdb) run ^M
Starting program: jit-reader ^M
gdb/jit.c:1247: internal-error: jit_event_handler: \
  Assertion `jiter->jiter_data != nullptr' failed.^M
...

Fix this by handling the
jit_bp_sym.objfile->separate_debug_objfile_backlink != nullptr case in
handle_jit_event.

Tested on x86_64-linux.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29277