Fix #70078: XSL callbacks with nodes as parameter leak memory

Fix #70078: XSL callbacks with nodes as parameter leak memory

The fix for bug #49634 solved a double-free by copying the node with
`xmlDocCopyNodeList()`, but the copied node is later freed by calling
`xmlFreeNode()` instead of `xmlFreeNodeList()`, thus leaking memory.
However, there is no need to treat the node as node list, i.e. to copy
also the node's siblings; just creating a recursive copy of the node
with `xmlDocCopyNode()` is sufficient, while that also avoids the leak.