Fix DAP stackTrace through frames without debuginfo

The DAP stackTrace implementation did not fully account for frames
without debuginfo.  Attemping this would yield a result like:

{"request_seq": 5, "type": "response", "command": "stackTrace", "success": false, "message": "'NoneType' object has no attribute 'filename'", "seq": 11}

This patch fixes the problem by adding another check for None.

gdb/python/lib/gdb/dap/bt.py

@@ -69,7 +69,7 @@ def _backtrace(thread_id, levels, startFrame):
                 "instructionPointerReference": hex(current_frame.pc()),
             }
             sal = _safe_sal(current_frame)
-            if sal is not None:
+            if sal is not None and sal.symtab is not None:
                 newframe["source"] = {
                     "name": os.path.basename(sal.symtab.filename),
                     "path": sal.symtab.filename,