soundwire: fix double free of dangling pointer

clang static analysis flags this problem

stream.c:844:9: warning: Use of memory after
  it is freed
        kfree(bus->defer_msg.msg->buf);
              ^~~~~~~~~~~~~~~~~~~~~~~

This happens in an error handler cleaning up memory
allocated for elements in a list.

	list_for_each_entry(m_rt, &stream->master_list, stream_node) {
		bus = m_rt->bus;

		kfree(bus->defer_msg.msg->buf);
		kfree(bus->defer_msg.msg);
	}

And is triggered when the call to sdw_bank_switch() fails.
There are a two problems.

First, when sdw_bank_switch() fails, though it frees memory it
does not clear bus's reference 'defer_msg.msg' to that memory.

The second problem is the freeing msg->buf. In some cases
msg will be NULL so this will dereference a null pointer.
Need to check before freeing.

Fixes: 99b8a5d608 ("soundwire: Add bank switch routine")
Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Link: https://lore.kernel.org/r/20200902202650.14189-1-trix@redhat.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

drivers/soundwire/stream.c

@@ -717,6 +717,7 @@ error:
 	kfree(wbuf);
 error_1:
 	kfree(wr_msg);
+	bus->defer_msg.msg = NULL;
 	return ret;
 }
 
@@ -840,9 +841,10 @@ static int do_bank_switch(struct sdw_stream_runtime *stream)
 error:
 	list_for_each_entry(m_rt, &stream->master_list, stream_node) {
 		bus = m_rt->bus;
-
-		kfree(bus->defer_msg.msg->buf);
-		kfree(bus->defer_msg.msg);
+		if (bus->defer_msg.msg) {
+			kfree(bus->defer_msg.msg->buf);
+			kfree(bus->defer_msg.msg);
+		}
 	}
 
 msg_unlock: