[SPARK-19180] [SQL] the offset of short should be 2 in OffHeapColumn

## What changes were proposed in this pull request?

the offset of short is 4 in OffHeapColumnVector's putShorts, but actually it should be 2.

## How was this patch tested?

unit test

Author: Yucai Yu <yucai.yu@intel.com>

Closes #16555 from yucai/offheap_short.

(cherry picked from commit ad0dadaa)
Signed-off-by: Davies Liu <davies.liu@gmail.com>