tracing/probes: Fix to return NULL and keep using current argc

Fix to return NULL and keep using current argc when there is
$argN and the BTF is not available.

Link: https://lore.kernel.org/all/168584574094.2056209.2694238431743782342.stgit@mhiramat.roam.corp.google.com/Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306030940.Cej2JoUx-lkp@intel.com/Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

kernel/trace/trace_probe.c

@@ -1273,7 +1273,8 @@ const char **traceprobe_expand_meta_args(int argc, const char *argv[],
 			trace_probe_log_err(0, NOSUP_BTFARG);
 			return (const char **)params;
 		}
-		return 0;
+		*new_argc = argc;
+		return NULL;
 	}
 	ctx->params = params;
 	ctx->nr_params = nr_params;