[xml] How to process errors from xmlReadMemory in multi-threaded context?

[Yuri <yuri rawbw com>]

I have many threads that call this function, they are all unaware of 
each other.
There is no context object that is passed to it, so no error handler can 
be attached to local context.
I can only pass an error handler to the global context: 
initGenericErrorDefaultFunc.
But this should be the global static function, and each my thread may 
have its own error handler.

How to process errors in multi-threaded way?

Yuri