vp9/common/vp9_reconinter: fix int sanitizer warnings

Unit Test: VP9/InvalidFileTest
implicit conversion from type 'int' of value -65536 (32-bit, signed) to
type 'int16_t' (aka 'short') changed the value to 0 (16-bit, signed)

BUG=webm:1615
BUG=webm:1645

Change-Id: I4ce0c6abf8b5bf43ee43e958ad75d9fa28b23eee
1 file changed