In file:
https://git.gnome.org/browse/libxml2/tree/parserInternals.c

Function : xmlParserAddNodeInfo

At line : 1985
 if ((pos < ctxt->node_seq.length) &&
        (ctxt->node_seq.buffer != NULL) &&
        (ctxt->node_seq.buffer[pos].node == info->node)) {
        ctxt->node_seq.buffer[pos] = *info;
    }
In above if condition[ ctxt->node_seq.buffer != NULL ] means value ctxt->node_seq.buffer can be NULL. Take this if condition as false

Now at line : 1992
 In else condition
 
   else {
        if (ctxt->node_seq.length + 1 > ctxt->node_seq.maximum) {

            ---Some COde --
        }
take if condition is false.

Again at line: 2018
    if (pos != ctxt->node_seq.length) {
            unsigned long i;

            for (i = ctxt->node_seq.length; i > pos; i--)
                ctxt->node_seq.buffer[i] = ctxt->node_seq.buffer[i - 1];
        }

Taking if condition as true
   
   ctxt->node_seq.buffer is dereferenced. As, it is NUll so may lead to crash.