lavc/avfft: fix RDFT wrapper stride

Per the lavu/tx docs:

> * For forward transforms (R2C), stride must be the spacing between two
> * samples in bytes. For inverse transforms, the stride must be set
> * to the spacing between two complex values in bytes.

The code did the reverse.
The stride parameter is currently not respected for RDFT transforms,
but has to be correct, for a potential future change.

libavcodec/avfft.c

@@ -158,7 +158,7 @@ RDFTContext *av_rdft_init(int nbits, enum RDFTransformType trans)
         return NULL;
     }
 
-    s->stride = (trans == DFT_C2R) ? sizeof(float) : sizeof(AVComplexFloat);
+    s->stride = (trans == DFT_C2R) ? sizeof(AVComplexFloat) : sizeof(float);
     s->len = 1 << nbits;
     s->inv = trans == IDFT_C2R;