Fix some unused variable warnings during Mesa build (only used in asserts)

Hey, while building Mesa I saw several warnings scroll by and this series addresses the first four I saw in my build log.

The changes have been compile-tested in a clean chroot (pbuilder) environment for amd64 and i386.

If you accept the patches, please apply them for me, as I don't have commit access for Mesa.

Cheers, Kai