Don't crash when there's no conceal frame

#include "vlc_decode.h"

#include "bitstream.h"

#include "scaling.h"

#include "log/log.h"

/* ====================================================================== /

Function : ConcealTexture_I()

****************************************************************************/

void CopyVopMB(Vop *curr, uint8 *prevFrame, int mbnum, int width_Y, int height)

{

    if (curr == NULL || prevFrame == NULL) {

        ALOGE("b/24630158");

        return;

    }

    int width_C = width_Y >> 1;

    int row = MB_SIZE;

    uint8              *y1, *y2, *u1, *u2, *v1, *v2;