vpx_dsp/x86/highbd_idct4x4_add_sse2: fix int sanitizer warnings

implicit conversion from type 'int' of value 49161 (32-bit, signed) to
type 'int16_t' (aka 'short') changed the value to -16375 (16-bit,
signed)

BUG=webm:1615

Change-Id: I3f18283609ac2ce365202a63ef61a47eb00c155b
1 file changed