Fix NullPointerException when checking if storage exists

app/src/main/java/org/schabi/newpipe/streams/io/StoredFileHelper.java

@@ -285,7 +285,7 @@ public class StoredFileHelper implements Serializable {
     }
 
     public boolean existsAsFile() {
-        if (source == null) {
+        if (source == null || (docFile == null && ioFile == null)) {
             return false;
         }
 

app/src/main/java/us/shandian/giga/get/DownloadMission.java

@@ -664,7 +664,7 @@ public class DownloadMission extends Mission {
      * @return {@code true}, if storage is invalid and cannot be used
      */
     public boolean hasInvalidStorage() {
-        return errCode == ERROR_PROGRESS_LOST || storage == null || storage.isInvalid() || !storage.existsAsFile();
+        return errCode == ERROR_PROGRESS_LOST || storage == null || !storage.existsAsFile();
     }
 
     /**