Avoid using uninitialized "driver" variable in driver creation error path
Avoid using uninitialized "driver" variable in driver creation error path

Right now, driver creation tries to use the driver variable even if it failed to create it.

Closes #305

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/305 from google:ci-fix b019722d536dc20580942478b022daaa97d4fac1
PiperOrigin-RevId: 287416827

iree-org/iree,iree-org/iree,iree-org/iree,iree-org/iree,iree-org/iree,iree-org/iree,google/iree,google/iree,google/iree,iree-org/iree,google/iree,google/iree,google/iree,google/iree