Fix a rare NPE crash in Android 2.x

See http://crashes.to/s/5ee30776a5d

Author: robUx4

AndroidAsync/src/com/koushikdutta/async/AsyncServer.java

@@ -727,6 +727,9 @@ private static void runLoop(final AsyncServer server, final SelectorWrapper sele
                 }
             }
         }
+        catch (NullPointerException e) {
+            throw new AsyncSelectorException(e);
+        }
         catch (IOException e) {
             throw new AsyncSelectorException(e);
         }