avcodec/sanm: Check extradata_size before allocations

Fixes: Leaks
Fixes: 15349/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_SANM_fuzzer-5102530557640704

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

libavcodec/sanm.c

@@ -491,6 +491,11 @@ static av_cold int decode_init(AVCodecContext *avctx)
 
     ctx->avctx   = avctx;
     ctx->version = !avctx->extradata_size;
+    // early sanity check before allocations to avoid need for deallocation code.
+    if (!ctx->version && avctx->extradata_size < 1026) {
+        av_log(avctx, AV_LOG_ERROR, "Not enough extradata.\n");
+        return AVERROR_INVALIDDATA;
+    }
 
     avctx->pix_fmt = ctx->version ? AV_PIX_FMT_RGB565 : AV_PIX_FMT_PAL8;
 
@@ -506,11 +511,6 @@ static av_cold int decode_init(AVCodecContext *avctx)
     if (!ctx->version) {
         int i;
 
-        if (avctx->extradata_size < 1026) {
-            av_log(avctx, AV_LOG_ERROR, "Not enough extradata.\n");
-            return AVERROR_INVALIDDATA;
-        }
-
         ctx->subversion = AV_RL16(avctx->extradata);
         for (i = 0; i < PALETTE_SIZE; i++)
             ctx->pal[i] = 0xFFU << 24 | AV_RL32(avctx->extradata + 2 + i * 4);