Fix error on invalid elements, roots

Closes rehypejs/rehype-raw#7.

Author: wooorm

index.js

@@ -100,9 +100,14 @@ function wrap(tree, file) {
   }
 
   function all(nodes) {
-    var length = nodes.length;
+    var length = 0;
     var index = -1;
 
+    /* istanbul ignore else - invalid nodes, see wooorm/rehype-raw#7. */
+    if (nodes) {
+      length = nodes.length;
+    }
+
     while (++index < length) {
       one(nodes[index]);
     }