diracdec: fix idwt_stride calculation in bytes

The transformation to bytes must happen after alignment to get the same
resulting pointers as before.

This fixes segmentation faults in the assembler code.

The regression was introduced in commit 9553689.

Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/diracdec.c

@@ -941,7 +941,7 @@ static void init_planes(DiracContext *s)
         p->height      = s->seq.height >> (i ? s->chroma_y_shift : 0);
         p->idwt_width  = w = CALC_PADDING(p->width , s->wavelet_depth);
         p->idwt_height = h = CALC_PADDING(p->height, s->wavelet_depth);
-        p->idwt_stride = FFALIGN(p->idwt_width << (1 + s->pshift), 8);
+        p->idwt_stride = FFALIGN(p->idwt_width, 8) << (1 + s->pshift);
 
         for (level = s->wavelet_depth-1; level >= 0; level--) {
             w = w>>1;