Fix debug assertion not covering both parts of the debug statement

[Elabajaba]

Description
Trivial fix to make wgpu build again when debug_assertions aren't enabled

Testing
cargo build --release now works

[kristoff3r]

Oops, sorry about that. I was sure that debug_assert_eq would disappear completely on release, but apparently not.

[ErichDonGubler]

Ack, thanks!

[jimblandy]

Oops, sorry about that. I was sure that debug_assert_eq would disappear completely on release, but apparently not.

Yeah, the key detail is this bit, from the debug_assert_eq! documentation:

The result of expanding debug_assert_eq! is always type checked.

So even when not(debug_assertions), the macro expansion still includes the expressions, just not in such a way that they'll be evaluated. This is to prevent mistakes symmetrical to the one here, in which users who are in the habit of doing release builds don't notice that their assertions are garbage until they happen to do a debug build.