Avoid restore download null

app/src/main/java/com/hippo/ehviewer/preference/RestoreDownloadPreference.java

@@ -90,6 +90,9 @@ public class RestoreDownloadPreference extends TaskPreference {
             try {
                 is = siFile.openInputStream();
                 SpiderInfo spiderInfo = SpiderInfo.read(is);
+                if (spiderInfo == null) {
+                    return null;
+                }
                 long gid = spiderInfo.gid;
                 if (mManager.containDownloadInfo(gid)) {
                     return null;

app/src/main/java/com/hippo/ehviewer/spider/SpiderInfo.java

@@ -94,6 +94,7 @@ public class SpiderInfo {
         }
     }
 
+    @Nullable
     @SuppressWarnings("InfiniteLoopStatement")
     public static SpiderInfo read(@Nullable InputStream is) {
         if (null == is) {