Fix filename filtering

Android seems to think different about the \w regex flag.
Also prevent empty names by replacing it with an underscore intead of a space.

Clover/app/src/main/java/org/floens/chan/utils/ImageSaver.java

@@ -169,7 +169,7 @@ public class ImageSaver {
     }
 
     private static String filterName(String name) {
-        return name.replaceAll("[^\\w.]", "");
+        return name.replaceAll("[^a-zA-Z0-9.]", "_");
     }
 
     private static File findUnused(File start, boolean isDir) {