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.