[xml] parent pointer

[Zhigang Chen <zhigangc gmail com>]

Hi all

Occasionally, I have seen some nodes have its parent pointer set to -1. Also they seem to have been freed 
implicitly, because (1) freeing them explicitly would throw error, (2) not freeing them does not incur memory 
leaks as reported by xmlMemBlocks.

I am wondering where in the source code the parent pointer is set to -1, and why it's not set to 0.

Thanks

-
Z