Fix for native crash on image decode OOM

When decoding an image with scaling, if the allocation of the bitmap
data for the scaled bitmap failed, we were just ignoring it and going
on. This was yielding strange native crashes (bug 7196860 and bug
7175536). This patch checks whether the allocation succeeds, and returns
a null bitmap if not.

Of course, if the app really is OOM because it's allocated too many
bitmaps, it'll still get the OOME, but that's a lot nicer than a native
crash or memory corruption.

Change-Id: I8384059ab11c2ab9e93e283b9438d79e6709b7b1