routeros_firmware: Fix parsing mikrotik web page

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

@@ -18,7 +18,11 @@ if [ $# -gt 0 ]; then
 fi
 
 packageurl="$(curl -sSf "https://mikrotik.com/download" | \
-    sed -n 's/.*<a href="\(.*routeros-[0-9\.]*-arm\.npk\)">.*/\1/p')"
+    sed -n 's/.*<a href="\(.*routeros-[0-9\.]*-arm\.npk\)"[> ].*/\1/p')"
+if [ -z "$packageurl" ]; then
+    echo "ERR: Got empty package URL, exiting" 1>&2
+    exit 1
+fi
 packagename="$(basename "$packageurl")"
 if [ -f "$packagename" ]; then
     "$verbose" && echo "Already up to date"