usd: Can't call usd_FileStandard* with NULL

It doesn't make sense to call usd_FileStandard{Input,Output} with
a NULL usd_handle_t (and doing so would crash later in the
function), so don't check for attempts to do so.

Caught by clang-analyzer

Change-Id: I03c0b627056108fe0d6f78d8028323cc4ed74758
Reviewed-on: http://gerrit.openafs.org/9151
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>