routeros_firmware: Give error if checksum fetch fails

roles/routeros_firmware/files/download-routeros-firmware.sh

@@ -31,6 +31,10 @@ fi
 
 checksum="$(curl -sSf "https://mikrotik.com/download" | \
 	sed -n 's/.*routeros-[0-9\.]*-arm\.npk<\/td>.*<td>SHA256<\/td><td>\(.*\)<\/td>.*/\1/p')"
+if [ -z "$checksum" ]; then
+    echo "ERR: Failed to determine package checksum" 1>&2
+    exit 1
+fi
 
 echo "Downloading new package '${packagename}'"
 trap 'rm -f -- "${packagename}.tmp"' EXIT