dds: limit 4 bpp handling to AV_PIX_FMT_PAL8

This fixes NULL pointer dereferencing for formats, where frame->data[1]
is not allocated.

The problem was introduced in commit
257fbc3a.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

libavcodec/dds.c

@@ -680,7 +680,7 @@ static int dds_decode(AVCodecContext *avctx, void *data,
         /* Use the decompress function on the texture, one block per thread. */
         ctx->tex_data = gbc->buffer;
         avctx->execute2(avctx, decompress_texture_thread, frame, NULL, ctx->slice_count);
-    } else if (!ctx->paletted && ctx->bpp == 4) {
+    } else if (!ctx->paletted && ctx->bpp == 4 && avctx->pix_fmt == AV_PIX_FMT_PAL8) {
         uint8_t *dst = frame->data[0];
         int x, y, i;