Gallery2:fix NPE when cannot load image in FilterShowActivity

When DRM image file can not be loaded, it still uses the
load-failed pointer(null) to get information. The
NullPointerException causes the unexpected result.

When load image failed, return directly from onPostExecute().

CRs-Fixed: 516899

Change-Id: Ib8f6903941aca8ef3f6b8a128fac9551aa03db27