avcodec/agm: Test remaining data in decode_raw_intra_rgb()

Fixes: Timeout (270sec -> 25ms)
Fixes: 20485/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_AGM_fuzzer-5636954207289344

Found-by: continuous fuzzing process https://github.com/google/oss-fuzz/tree/master/projects/ffmpeg
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/agm.c

@@ -573,13 +573,16 @@ static int decode_raw_intra_rgb(AVCodecContext *avctx, GetByteContext *gbyte, AV
     uint8_t *dst = frame->data[0] + (avctx->height - 1) * frame->linesize[0];
     uint8_t r = 0, g = 0, b = 0;
 
+    if (bytestream2_get_bytes_left(gbyte) < 3 * avctx->width * avctx->height)
+        return AVERROR_INVALIDDATA;
+
     for (int y = 0; y < avctx->height; y++) {
         for (int x = 0; x < avctx->width; x++) {
-            dst[x*3+0] = bytestream2_get_byte(gbyte) + r;
+            dst[x*3+0] = bytestream2_get_byteu(gbyte) + r;
             r = dst[x*3+0];
-            dst[x*3+1] = bytestream2_get_byte(gbyte) + g;
+            dst[x*3+1] = bytestream2_get_byteu(gbyte) + g;
             g = dst[x*3+1];
-            dst[x*3+2] = bytestream2_get_byte(gbyte) + b;
+            dst[x*3+2] = bytestream2_get_byteu(gbyte) + b;
             b = dst[x*3+2];
         }
         dst -= frame->linesize[0];