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: Icd368afcbed80556de0940e562dbcca42c53c849
CRs-Fixed: 525081