Fixed xpath crashes discovered by fuzzing

Added fuzz code for xpath
Test: added negative xpath tests

test/fuzz/xpath/runfuzz.sh

@@ -0,0 +1,34 @@
+#!/usr/bin/env bash
+# Run a fuzzing test using american fuzzy lop
+set -eux
+
+if [ $# -ne 0 ]; then 
+    echo "usage: $0\n"
+    exit 255
+fi
+
+APPNAME=example
+xml=example.xml
+
+
+cat <<EOF > $xml
+<table xmlns="urn:example:clixon">
+   <parameter>
+      <name>x</name>
+      <value>42</value>
+   </parameter>
+</table>
+EOF
+
+MEGS=500 # memory limit for child process (50 MB)
+
+# remove input and input dirs
+#test ! -d input || rm -rf input
+test ! -d output || rm -rf output
+
+# create if dirs dont exists
+#test -d input || mkdir input
+test -d output || mkdir output
+
+# Run script 
+afl-fuzz -i input -o output -m $MEGS -- clixon_util_xpath -f $xml -n ex:urn:example:clixon -y /usr/local/share/clixon/clixon-example@2022-11-01.yang  -Y /usr/local/share/clixon