Surface: return early from dequeueBuffer on failure

In the function

```
Surface::dequeueBuffer(sp<GraphicBuffer>* buffer, sp<Fence>* outFence)
```

we'd always try to map the uninitialized android_native_buffer_t to a
GraphicBuffer.  This would lead to an NPE in the refcounting code
sometimes.

Now, we'll stop short and return early on error.

Bug: 414442592
Flag: EXEMPT small bugfix
Test: new test
Change-Id: I592a94f356d289be0b49696c428b5ba70f9c9803