avcodec/avpacket: Respect payload offset in av_grow_packet

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

libavcodec/avpacket.c

@@ -110,24 +110,38 @@ int av_grow_packet(AVPacket *pkt, int grow_by)
 {
     int new_size;
     av_assert0((unsigned)pkt->size <= INT_MAX - AV_INPUT_BUFFER_PADDING_SIZE);
-    if (!pkt->size)
-        return av_new_packet(pkt, grow_by);
     if ((unsigned)grow_by >
         INT_MAX - (pkt->size + AV_INPUT_BUFFER_PADDING_SIZE))
         return -1;
 
     new_size = pkt->size + grow_by + AV_INPUT_BUFFER_PADDING_SIZE;
     if (pkt->buf) {
-        int ret = av_buffer_realloc(&pkt->buf, new_size);
-        if (ret < 0)
+        size_t data_offset;
+        uint8_t *old_data = pkt->data;
+        if (pkt->data == NULL) {
+            data_offset = 0;
+            pkt->data = pkt->buf->data;
+        } else {
+            data_offset = pkt->data - pkt->buf->data;
+            if (data_offset > INT_MAX - new_size)
+                return -1;
+        }
+
+        if (new_size + data_offset > pkt->buf->size) {
+            int ret = av_buffer_realloc(&pkt->buf, new_size + data_offset);
+            if (ret < 0) {
+                pkt->data = old_data;
                 return ret;
+            }
+            pkt->data = pkt->buf->data + data_offset;
+        }
     } else {
         pkt->buf = av_buffer_alloc(new_size);
         if (!pkt->buf)
             return AVERROR(ENOMEM);
-        memcpy(pkt->buf->data, pkt->data, FFMIN(pkt->size, pkt->size + grow_by));
-    }
+        memcpy(pkt->buf->data, pkt->data, pkt->size);
         pkt->data = pkt->buf->data;
+    }
     pkt->size += grow_by;
     memset(pkt->data + pkt->size, 0, AV_INPUT_BUFFER_PADDING_SIZE);