Initialize fallback ref

* devstack-vm-gate-wrap.sh: Set the fallback reference to an empty
string so it won't reuse a prior loop iteration's value.

Change-Id: I118a7b6488695ad7fe1b80d418548244bdf24452

devstack-vm-gate-wrap.sh

@@ -108,6 +108,7 @@ function setup_workspace {
       # Ensure that we don't have stale remotes around
       git remote prune origin
       # See if this project has this branch, if not, use master
+      FALLBACK_ZUUL_REF=""
       if ! git branch -a |grep remotes/origin/$BRANCH>/dev/null; then
         BRANCH=master
         FALLBACK_ZUUL_REF=$(echo $ZUUL_REF | sed -e "s,$ZUUL_BRANCH,master,")