url-lib: check for ctorrent binary before adding the url handler

modules.d/45url-lib/url-lib.sh

@@ -110,7 +110,9 @@ ctorrent_fetch_url() {
     fi
     if [ -z "$2" ]; then echo "$outloc" ; fi
 }
-add_url_handler ctorrent_fetch_url torrent
+
+command -v ctorrent >/dev/null \
+    && add_url_handler ctorrent_fetch_url torrent
 
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