Fix T90564: Crash when linking 2 node inputs

Caused by {rB37570a73170e}.

Above commit wasnt taking into account that at this point the link could
still be NULL.

Maniphest Tasks: T90564

Differential Revision: https://developer.blender.org/D12180

source/blender/blenkernel/intern/node.cc

@@ -2364,7 +2364,7 @@ bNodeLink *nodeAddLink(
     ntree->update |= NTREE_UPDATE_LINKS;
   }
 
-  if (link->tosock->flag & SOCK_MULTI_INPUT) {
+  if (link != nullptr && link->tosock->flag & SOCK_MULTI_INPUT) {
     link->multi_input_socket_index = node_count_links(ntree, link->tosock) - 1;
   }