Fix a mistake in commit 2ce5fc4a3e that caused a crash when detaching
node links from input sockets.
When a link is detached from an input socket, `nodeRemLink` nulls the
`link` pointer of the socket.
So before the next update inputs are linked but don't have a valid `link`
pointer causing the crash, when trying to access the link in
`std_node_socket_draw`.
The introduced check avoids the crash and is more correct since it
doesn't just check one link for multi-input sockets.
Pull Request: https://projects.blender.org/blender/blender/pulls/108623
e6a8c45fd9