Uncontrolled Recursion in libxml2

Summary

Uncontrolled recursion in XPath evaluation in libxml2 versions through 2.9.14 allow a local attacker to cause a Stack Overflow via crafted expressions. XPath processing functions `xmlXPathRunEval`, `xmlXPathCtxtCompile`, and `xmlXPathEvalExpr` were resetting recursion depth to zero before making potentially recursive calls. When such functions were called recursively this could allow for uncontrolled recursion and lead to a Stack Overflow. These functions now preserve recursion depth across recursive calls, allowing recursion depth to be controlled.