make sure c-doc upload dirs exist even if branch name has / in it

ci_scripts/ci_upload.sh

@@ -22,6 +22,7 @@ rsync -avz \
   delta@py.delta.chat:build/${BRANCH}
 
 # C docs to c.delta.chat
+ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null delta@c.delta.chat mkdir -p build-c/${BRANCH}
 rsync -avz \
   -e "ssh -o StrictHostKeyChecking=no -o UserKnownHostsFile=/dev/null" \
   "$DOXYDOCDIR/html/" \