Fix nullptr deref with XPath logic ops
If the XPath stack is corrupted, for example by a misbehaving extension function, the "and" and "or" XPath operators could dereference NULL pointers. Check that the XPath stack isn't empty and optimize the logic operators slightly. Closes: #5 Also see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=901817 https://bugzilla.redhat.com/show_bug.cgi?id=1595985 This is CVE-2018-14404. Thanks to Guy Inbar for the report.
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