vp9/decoder/vp9_detokenize: fix int sanitizer warnings

From unit test: VP9MultiThreaded/InvalidFileTest
implicit conversion from type 'int' of value 83144 (32-bit, signed) to
type 'tran_low_t' (aka 'short') changed the value to 17608 (16-bit,
 signed)

 BUG=webm:1615
 BUG=webm:1648

Change-Id: I4170494c328596ace66432c8563c55f31745cf76
1 file changed