Gallery2: Fix Gallery Crash when click Information menu.

In UpdatePreviewTask,the condition in the doInBackground
is not correct which cause the NullPointerException happened.

Only judge the buffer.getProducer() before it is used.

CRs-Fixed: 619081

Conflicts:
	src/com/android/gallery3d/filtershow/pipeline/UpdatePreviewTask.java

Change-Id: Iac3152328abeb72002785cfa2eb28830ea87f439
Merge "Gallery2: Fix Gallery Crash when click Information menu." into du44