vpx_dsp/quantize: fix int sanitizer warnings

From unit test: AVX/VP9QuantizeTest; SSSE3/VP9QuantizeTest ...
implicit conversion from type 'int' of value -139812 (32-bit, signed)
to type 'tran_low_t' (aka 'short') changed the value to -8740 (16-bit,
 signed)

BUG=webm:1615

Change-Id: I730946ac6c7a250dcbcfd8a2712c0f1150ddb4fd
1 file changed