Clean up DataFetchers even if DecodeJobs are cancelled prior to run().

Previously some of the logic in run() might null out the data fetcher
instance variable, even if it had been set when run() was first called.
As a result, the data fetcher would not be cleared. Now the initial 
data fetcher is held on to as a local variable so the instance variable
can be freed without affecting the behavior of cleanup().

Progress towards #1996, #2352

library/src/main/java/com/bumptech/glide/load/engine/DecodeJob.java

@@ -18,6 +18,7 @@
 import com.bumptech.glide.load.engine.cache.DiskCache;
 import com.bumptech.glide.load.resource.bitmap.Downsampler;
 import com.bumptech.glide.util.LogTime;
+import com.bumptech.glide.util.Preconditions;
 import com.bumptech.glide.util.Synthetic;
 import com.bumptech.glide.util.pool.FactoryPools.Poolable;
 import com.bumptech.glide.util.pool.StateVerifier;
@@ -218,6 +219,9 @@ public void run() {
     // swallows all otherwise fatal exceptions, this will at least make it obvious to developers
     // that something is failing.
     TraceCompat.beginSection("DecodeJob#run");
+    // Methods in the try statement can invalidate currentFetcher, so set a local variable here to
+    // ensure that the fetcher is cleaned up either way.
+    DataFetcher<?> localFetcher = currentFetcher;
     try {
       if (isCancelled) {
         notifyFailed();
@@ -238,8 +242,11 @@ public void run() {
         throw e;
       }
     } finally {
-      if (currentFetcher != null) {
-        currentFetcher.cleanup();
+      Preconditions.checkArgument(
+          localFetcher == null || currentFetcher == null || localFetcher.equals(currentFetcher),
+          "Fetchers don't match!, old: " + localFetcher + " new: " + currentFetcher);
+      if (localFetcher != null) {
+        localFetcher.cleanup();
       }
       TraceCompat.endSection();
     }