enable integer overflow checks on release profile

This will harden crosvm against potential integer overflow leading to
unsafe behavior in unsafe blocks.

TEST=cargo test --release
BUG=None

Change-Id: Ie750906d04c6ef8cb9400b976827c2abeab2ab56
Reviewed-on: https://chromium-review.googlesource.com/1282064
Commit-Ready: Zach Reizner <zachr@chromium.org>
Tested-by: Zach Reizner <zachr@chromium.org>
Reviewed-by: Dylan Reid <dgreid@chromium.org>
diff --git a/Cargo.toml b/Cargo.toml
index 7d17364..c419574 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -6,6 +6,7 @@
 [profile.release]
 lto = true
 panic = 'abort'
+overflow-checks = true
 
 [workspace]
 members = ["9s"]