avcodec/hevcdec: Fix undefined memcpy()

There is likely a better way to fix this, this is mainly to show the problem

Fixes: MC within same frame resulting in overlapping memcpy()
Fixes: 60189/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-4992746590175232
Fixes: 61753/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-5022150806077440
Fixes: 58062/clusterfuzz-testcase-minimized-ffmpeg_AV_CODEC_ID_HEVC_fuzzer-4717458841010176

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

libavcodec/hevcdec.c

@@ -1563,7 +1563,8 @@ static void luma_mc_uni(HEVCLocalContext *lc, uint8_t *dst, ptrdiff_t dststride,
 
     if (x_off < QPEL_EXTRA_BEFORE || y_off < QPEL_EXTRA_AFTER ||
         x_off >= pic_width - block_w - QPEL_EXTRA_AFTER ||
-        y_off >= pic_height - block_h - QPEL_EXTRA_AFTER) {
+        y_off >= pic_height - block_h - QPEL_EXTRA_AFTER ||
+        ref == s->frame) {
         const ptrdiff_t edge_emu_stride = EDGE_EMU_BUFFER_STRIDE << s->ps.sps->pixel_shift;
         int offset     = QPEL_EXTRA_BEFORE * srcstride       + (QPEL_EXTRA_BEFORE << s->ps.sps->pixel_shift);
         int buf_offset = QPEL_EXTRA_BEFORE * edge_emu_stride + (QPEL_EXTRA_BEFORE << s->ps.sps->pixel_shift);
@@ -1713,6 +1714,7 @@ static void chroma_mc_uni(HEVCLocalContext *lc, uint8_t *dst0,
     intptr_t my          = av_mod_uintp2(mv->y, 2 + vshift);
     intptr_t _mx         = mx << (1 - hshift);
     intptr_t _my         = my << (1 - vshift);
+    int emu              = src0 == s->frame->data[1] || src0 == s->frame->data[2];
 
     x_off += mv->x >> (2 + hshift);
     y_off += mv->y >> (2 + vshift);
@@ -1720,7 +1722,8 @@ static void chroma_mc_uni(HEVCLocalContext *lc, uint8_t *dst0,
 
     if (x_off < EPEL_EXTRA_BEFORE || y_off < EPEL_EXTRA_AFTER ||
         x_off >= pic_width - block_w - EPEL_EXTRA_AFTER ||
-        y_off >= pic_height - block_h - EPEL_EXTRA_AFTER) {
+        y_off >= pic_height - block_h - EPEL_EXTRA_AFTER ||
+        emu) {
         const int edge_emu_stride = EDGE_EMU_BUFFER_STRIDE << s->ps.sps->pixel_shift;
         int offset0 = EPEL_EXTRA_BEFORE * (srcstride + (1 << s->ps.sps->pixel_shift));
         int buf_offset0 = EPEL_EXTRA_BEFORE *