avcodec/avdct: Clear IDCTDSPContext context

Fixes use of uninitialized variable and segfault

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit b82825eba8)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/avdct.c

@@ -100,7 +100,7 @@ int avcodec_dct_init(AVDCT *dsp)
 
 #if CONFIG_IDCTDSP
     {
-        IDCTDSPContext idsp;
+        IDCTDSPContext idsp = {0};
         ff_idctdsp_init(&idsp, avctx);
         COPY(idsp, idct);
         COPY(idsp, idct_permutation);