minigbm: add validation during import

minigbm: add validation during import

A common cause of bugs and security exploits are out-of-bounds
memory accesses. Let's add some validation when importing the
buffer to protect against this.

BUG=b:65566935
TEST=Boot Android and play games on Eve

Change-Id: I557f9b9849f267f5c87bb29648119b1c29695cd3
Reviewed-on: https://chromium-review.googlesource.com/692624
Commit-Ready: Gurchetan Singh <gurchetansingh@chromium.org>
Tested-by: Gurchetan Singh <gurchetansingh@chromium.org>
Reviewed-by: Stéphane Marchesin <marcheu@chromium.org>