Add a null check in exifParser to avoid crash while setting Wallpaper.

For some images, while setting as Wallpaper, the src array in
System.arraycopy was null in exifParser, which leads to crash.
This change adds null check to the array used in System.arraycopy.
Also, a safe check is made to ensure start index is not negative.

Change-Id: I46a16b6943aaeeb2395e335bf3926988b881be7a