af_unix: Clean up error paths in unix_stream_connect().

The label order is weird in unix_stream_connect(), and all NULL checks
are unnecessary if reordered.

Let's clean up the error paths to make it easy to set a drop reason
for each path.

While at it, a comment with the old style is updated.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>