fftools/ffmpeg_enc: Initialize fd

Fixes: CID1520677 Uninitialized pointer read

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 462bd44b03)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg_enc.c

@@ -490,7 +490,7 @@ void enc_stats_write(OutputStream *ost, EncStats *es,
     AVRational  tbi = (AVRational){ 0, 1};
     int64_t    ptsi = INT64_MAX;
 
-    const FrameData *fd;
+    const FrameData *fd = NULL;
 
     if (frame ? frame->opaque_ref : pkt->opaque_ref) {
         fd   = (const FrameData*)(frame ? frame->opaque_ref->data : pkt->opaque_ref->data);