VAAPI-X11: Make sure the base pointer for vaMapBuffer is NULL at start

Set p_base to NULL before passing it to vaMapBuffer.

modules/codec/avcodec/vaapi_x11.c

@@ -958,7 +958,7 @@ static int CopyPictureToVAImage(vout_display_t *vd, picture_t *pic,
     assert(image->image_id != VA_INVALID_ID);
     assert(image->buf != VA_INVALID_ID);
 
-    void *p_base;
+    void *p_base = NULL;
     status = vaMapBuffer(sys->conn->p_display, image->buf, &p_base);
     if (status != VA_STATUS_SUCCESS)
     {
@@ -971,6 +971,7 @@ static int CopyPictureToVAImage(vout_display_t *vd, picture_t *pic,
                  image->format.bits_per_pixel, pic->format.i_bits_per_pixel);
         goto out_va_free;
     }
+    assert(p_base); /* should not happen */
 
     for (unsigned int i_plane = 0; i_plane < image->num_planes; i_plane++)
     {