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/ffmpegSigned-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/targa_y216dec.c

@@ -35,7 +35,8 @@ static int y216_decode_frame(AVCodecContext *avctx, void *data,
 {
     AVFrame *pic = data;
     const uint16_t *src = (uint16_t *)avpkt->data;
-    uint16_t *y, *u, *v, aligned_width = FFALIGN(avctx->width, 4);
+    uint16_t *y, *u, *v;
+    int aligned_width = FFALIGN(avctx->width, 4);
     int i, j, ret;
 
     if (avpkt->size < 4 * avctx->height * aligned_width) {