vp9_svc_layercontext.c: fix integer sanitizer warnings

implicit conversion from type 'int' of value -1
(32-bit, signed) to type 'uint8_t' (aka 'unsigned char') changed the
value to 255 (8-bit, unsigned

BUG=webm:1615

Change-Id: If507e73aea4dccd3914b6470f8d15db3b67300ce
1 file changed