vpx_dsp/x86/fwd_txfm_sse2: fix int sanitizer warnings

implicit conversion from type 'int' of value 32768 (32-bit, signed)
to type 'short' changed the value to -32768 (16-bit, signed)

BUG=webm:1615

Change-Id: I7cdba7f7e550f62fd3ac31574e49b1909b6ab054
1 file changed