Reject downloaded lists which are deemed truncated

Related feedback:
- https://www.reddit.com/r/uBlockOrigin/comments/hbpo86/

For unknown reasons at this point, it appears some users
end up with a truncated version of EasyList, leading to
improper blocking in uBO.

This commit adds a heuristic to discard a downloaded list
when its new size is more than 25% smaller than the
currently cached version.

src/js/assets.js

@@ -771,6 +771,22 @@ const getRemote = async function(assetKey) {
             ? await api.fetchFilterList(contentURL)
             : await api.fetchText(contentURL);
 
+        // https://www.reddit.com/r/uBlockOrigin/comments/hbpo86/
+        //   Server sent a truncated list? If the new list is smaller than the
+        //   currently cached one by more than 25%, assume a server error.
+        if (
+            stringIsNotEmpty(result.content) &&
+            typeof assetDetails.size === 'number' &&
+            assetDetails.size !== 0
+        ) {
+            const loss = 1 - result.content.length / assetDetails.size;
+            if ( loss > 0.25 ) {
+                result.statusCode = 206;
+                result.statusText = 'Partial Content';
+                result.content = '';
+            }
+        }
+
         // Failure
         if ( stringIsNotEmpty(result.content) === false ) {
             let error = result.statusText;
@@ -788,7 +804,10 @@ const getRemote = async function(assetKey) {
             content: result.content,
             url: contentURL
         });
-        registerAssetSource(assetKey, { error: undefined });
+        registerAssetSource(assetKey, {
+            error: undefined,
+            size: result.content.length
+        });
         return reportBack(result.content);
     }