avcodec/pngenc: Initialize fctl_chunk to 0

The structure is copied around and that triggers warnings if it is uninitialized
Fixes CID1322360
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/pngenc.c

@@ -817,7 +817,7 @@ static int encode_apng(AVCodecContext *avctx, AVPacket *pkt,
     int ret;
     int enc_row_size;
     size_t max_packet_size;
-    APNGFctlChunk fctl_chunk;
+    APNGFctlChunk fctl_chunk = {0};
 
     if (pict && avctx->codec_id == AV_CODEC_ID_APNG && s->color_type == PNG_COLOR_TYPE_PALETTE) {
         uint32_t checksum = ~av_crc(av_crc_get_table(AV_CRC_32_IEEE_LE), ~0U, pict->data[1], 256 * sizeof(uint32_t));