minigbm: Fix cast to avoid null pointer arithmetic

minigbm: Fix cast to avoid null pointer arithmetic

'external/minigbm/cros_gralloc/gralloc0/gralloc0.cc:417:50:
error: arithmetic on a null pointer treated as a cast from
integer to pointer is a GNU extension'

BUG=b:146515640
TEST=m gralloc.minigbm

Change-Id: I2e86b676224657bdb656a45db4cfa49e84b61c2d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/minigbm/+/2202518
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Auto-Submit: Jason Macnak <natsu@google.com>
Tested-by: Jason Macnak <natsu@google.com>
Commit-Queue: Jason Macnak <natsu@google.com>