Add sanity check to yang-tools release job

Change-Id: I62ad1acb7833288143ebeb4583ead77fbbe8b3b6
diff --git a/jjb/onos/onos-yang-tools-release-build.sh b/jjb/onos/onos-yang-tools-release-build.sh
index 1036e65..b02d429 100644
--- a/jjb/onos/onos-yang-tools-release-build.sh
+++ b/jjb/onos/onos-yang-tools-release-build.sh
@@ -24,5 +24,11 @@
 # get the sources
 git checkout $GERRIT_BRANCH
 
+# sanity check: does the tag already exist?
+if git rev-parse -q --verify "refs/tags/${YANGTOOLS_VERSION}"; then
+  echo "${YANGTOOLS_VERSION} tag already exists"
+  exit -1
+fi
+
 # run the release process
 tools/onos-yang-tools-build-release ${YANGTOOLS_VERSION} ${YANGTOOLS_NEXT_VERSION} ${GERRIT_BRANCH}