Only check signed-off-by in PRs

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/scripts/check-signed-off-by.sh b/scripts/check-signed-off-by.sh
index 1205521..aaf40a5 100755
--- a/scripts/check-signed-off-by.sh
+++ b/scripts/check-signed-off-by.sh
@@ -14,6 +14,10 @@
 
 # this retrieves the merge commit created by GH
 parents=(`git log -n 1 --format=%p HEAD`)
+branch=(`git rev-parse --abbrev-ref HEAD`)
+
+# Should only run in PR branches
+[[ ${branch} == "master" ]] && exit 0
 
 if [[ "${#parents[@]}" -ne 2 ]]; then
   echo "This PR's merge commit is missing a parent!"