avcodec/pngdec: Check deloco index more exactly

Fixes: out of array access:
Fixes: 48567/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_PNG_fuzzer-6716193709096960

Alternatively it should be possible to limit this to 3 plane RGB 8 /16bit to ensure the size is what it should be

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

libavcodec/pngdec.c

@@ -329,7 +329,7 @@ void ff_png_filter_row(PNGDSPContext *dsp, uint8_t *dst, int filter_type,
 static void deloco_ ## NAME(TYPE *dst, int size, int alpha) \
 { \
     int i; \
-    for (i = 0; i < size; i += 3 + alpha) { \
+    for (i = 0; i < size - 2; i += 3 + alpha) { \
         int g = dst [i + 1]; \
         dst[i + 0] += g; \
         dst[i + 2] += g; \