Fix exception thrown when a stock asset is removed

Related feedback:
- https://www.reddit.com/r/uMatrix/comments/ftebgz/

assets/assets.json

@@ -16,25 +16,6 @@
 			"assets/thirdparties/publicsuffix.org/list/effective_tld_names.dat"
 		]
 	},
-	"malware-0": {
-		"content": "filters",
-		"title": "Malware Domain List",
-		"contentURL": [
-			"https://www.malwaredomainlist.com/hostslist/hosts.txt",
-			"assets/thirdparties/www.malwaredomainlist.com/hostslist/hosts.txt"
-		]
-	},
-	"malware-1": {
-		"content": "filters",
-		"title": "Malware domains",
-		"contentURL": [
-			"https://mirror.cedia.org.ec/malwaredomains/justdomains",
-			"https://mirror1.malwaredomains.com/files/justdomains",
-			"assets/thirdparties/mirror1.malwaredomains.com/files/justdomains",
-			"assets/thirdparties/mirror1.malwaredomains.com/files/justdomains.txt"
-		],
-		"supportURL": "http://www.malwaredomains.com/"
-	},
 	"dpollock-0": {
 		"content": "filters",
 		"updateAfter": 11,

src/js/storage.js

@@ -552,12 +552,22 @@
     var usedCount = this.ubiquitousBlacklist.count;
     var duplicateCount = this.ubiquitousBlacklist.duplicateCount;
 
-    this.mergeHostsFileContent(details.content);
+    // https://www.reddit.com/r/uMatrix/comments/ftebgz/
+    //   Be ready to deal with a removed asset.
+
+    if ( typeof details.content === 'string' && details.content !== '' ) {
+        this.mergeHostsFileContent(details.content);
+    }
 
     usedCount = this.ubiquitousBlacklist.count - usedCount;
     duplicateCount = this.ubiquitousBlacklist.duplicateCount - duplicateCount;
 
-    let hostsFileMeta = this.liveHostsFiles.get(details.assetKey);
+    const hostsFileMeta = this.liveHostsFiles.get(details.assetKey);
+    if ( hostsFileMeta === undefined ) {
+        this.liveHostsFiles.delete(details.assetKey);
+        return;
+    }
+
     hostsFileMeta.entryCount = usedCount + duplicateCount;
     hostsFileMeta.entryUsedCount = usedCount;
 };