avcodec/targa_y216dec: Fix width type

Fixes out of array access
Fixes: 1376/clusterfuzz-testcase-minimized-6361794975105024

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/targets/ffmpeg
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
