Fix T53270: crash with multiscatter GGX after recent refactoring.

In fact this was an existing issue when exceeding the number of available
closure, but it's more common now that we set the number to 0 for shadows
and emission

intern/cycles/kernel/closure/alloc.h

@@ -45,7 +45,7 @@ ccl_device ccl_addr_space void *closure_alloc_extra(ShaderData *sd, int size)
 	int num_extra = ((size + sizeof(ShaderClosure) - 1) / sizeof(ShaderClosure));
 
 	if(num_extra > sd->num_closure_left) {
-		/* Remove previous closure. */
+		/* Remove previous closure if it was allocated. */
 		sd->num_closure--;
 		sd->num_closure_left++;
 		return NULL;