Fixed xpath crashes discovered by fuzzing

Added fuzz code for xpath
Test: added negative xpath tests

test/fuzz/xpath/README.md

@@ -0,0 +1,31 @@
+# Clixon xpath fuzzing
+
+This dir contains code for fuzzing clixon xpaths. 
+
+## Prereqs
+
+Install AFL, see [..](..)
+
+## Build
+
+Build clixon clixon_util_xpath statically with the afl-clang compiler:
+
+```
+  CC=/usr/bin/afl-clang-fast LINKAGE=static INSTALLFLAGS="" ./configure
+  make clean
+  cd lib
+  make
+  sudo make install
+  cd ../util
+  make clixon_util_xpath
+  sudo install clixon_util_xpath /usr/local/bin/ # some utils have complex dependencies
+```
+
+## Run tests
+
+Run the script `runfuzz.sh` to run one test with a yang spec and an input string, eg:
+```
+  ./runfuzz.sh
+```
+
+After (or during) the test, investigate results in the output dir.

test/fuzz/xpath/input/1.xpath

@@ -0,0 +1 @@
+/ex:table[ex:parameter='x']

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