avcodec/dxva2_h264: Fix "may be used uninitialized" warnings

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

libavcodec/dxva2_h264.c

@@ -298,9 +298,9 @@ static int commit_bitstream_and_slice_buffer(AVCodecContext *avctx,
     const H264Picture *current_picture = h->cur_pic_ptr;
     struct dxva2_picture_context *ctx_pic = current_picture->hwaccel_picture_private;
     DXVA_Slice_H264_Short *slice = NULL;
-    void     *dxva_data_ptr;
+    void     *dxva_data_ptr = NULL;
     uint8_t  *dxva_data, *current, *end;
-    unsigned dxva_size;
+    unsigned dxva_size = 0;
     void     *slice_data;
     unsigned slice_size;
     unsigned padding;