Crash fix when reading a Bitmap requires too much memory

Here is the stack trace I get (usually with animated GIF)

```
java.lang.OutOfMemoryError
    at java.nio.HeapByteBuffer.<init>(HeapByteBuffer.java:48)
    at java.nio.ReadWriteHeapByteBuffer.<init>(ReadWriteHeapByteBuffer.java:49)
    at java.nio.ByteBuffer.allocate(ByteBuffer.java:52)
    at com.koushikdutta.async.ByteBufferList.obtain(ByteBufferList.java:479)
    at com.koushikdutta.async.ByteBufferList.read(ByteBufferList.java:232)
    at com.koushikdutta.async.ByteBufferList.getAll(ByteBufferList.java:210)
    at com.koushikdutta.ion.LoadBitmap$1.run(LoadBitmap.java:54)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
    at java.lang.Thread.run(Thread.java:856)
```