Gallery2: fix issue in readTag when parse Exif

When read some Tag of jpeg images,the offset is less than
the IFD0 offset position,then try to copy data above IFD0
as Tag value no matter whether data exists above IFD0,so
null exception occurs in System.arraycopy.This issue can
effect user in two aspects:
1.black thumbnail for some jpeg images in ALbumSetPage
and ALbumPage.
2.force close happens when get details of some jpeg images.

We set offset for this Tag not value on above situation.

Change-Id: Idc5537f751638f691822f6fe5300a486529e263f
CRs-Fixed: 887654