gtx: Return failure if gtx_Init can't start thread

If LWP_CreateProcess fails in gtx_Init return NULL, rather than
silently swallowing the failure.

Caught by clang-analyzer

Change-Id: I5db5667f9dae3f405e88cf9c57e6b8a069ea28a3
Reviewed-on: http://gerrit.openafs.org/9236
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>