NullPointerException: Attempt to invoke virtual method 'int com.b.a.i.ordinal()' on a null object reference

This is a bug reported by users. Seems to be a random bug so far. I wasn't be able to get the bug reproduced yet. However, I get couple of bug reports on Firebase crash report every day. So I think it is worth to report the bug here and hopefully a fix soon. Thanks.

**Glide Version**: 4.3.1

**Integration libraries**: Android Architecture Components v1.0.0

**Device/Android Version**: Various devices and Android versions.

**Issue details / Repro steps / Use case background**:

**Glide load line / `GlideModule` (if any) / list Adapter code (if any)**:
```java
Glide.with...
```

**Layout XML**:
```xml

Start by reviewing DecodeJob.java around getPriority (line 204) and compareTo (line 196), then trace how work is enqueued through GlideExecutor.java and EngineJob.java. The issue provides no reproduction steps or test name; done means identifying the null-priority path, preventing the crash, and adding regression coverage for it.

Written by the indexing model from the issue text.