avcodec/dct: use av_freep(), do not leave stale pointers in memory

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dct.c

@@ -193,7 +193,7 @@ av_cold int ff_dct_init(DCTContext *s, int nbits, enum DCTTransformType inverse)
         s->csc2   = av_malloc_array(n / 2, sizeof(FFTSample));
 
         if (ff_rdft_init(&s->rdft, nbits, inverse == DCT_III) < 0) {
-            av_free(s->csc2);
+            av_freep(&s->csc2);
             return -1;
         }
 
@@ -218,5 +218,5 @@ av_cold int ff_dct_init(DCTContext *s, int nbits, enum DCTTransformType inverse)
 av_cold void ff_dct_end(DCTContext *s)
 {
     ff_rdft_end(&s->rdft);
-    av_free(s->csc2);
+    av_freep(&s->csc2);
 }