Fix overflow issue in 16x16 quantization SSSE3

The 16x16 transform unit test suggested that the peak coefficient
value can reach 32639. This could cause potential overflow issue
in the SSSE3 implmentation of 16x16 block quantization. This commit
fixes this issue by replacing addition with saturated addition.

Change-Id: I6d5bb7c5faad4a927be53292324bd2728690717e