fftools/ffmpeg_enc: Initialize Decoder

Fixes: CID1591439 Uninitialized pointer read

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit 935d9a5712)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

fftools/ffmpeg_enc.c

@@ -171,7 +171,7 @@ int enc_open(void *opaque, const AVFrame *frame)
     InputStream *ist = ost->ist;
     Encoder              *e = ost->enc;
     AVCodecContext *enc_ctx = ost->enc_ctx;
-    Decoder            *dec;
+    Decoder            *dec = NULL;
     const AVCodec      *enc = enc_ctx->codec;
     OutputFile          *of = ost->file;
     FrameData *fd;