libafs: don't free the NULL we get from a failed allocation

libafs: don't free the NULL we get from a failed allocation

In extractPioctlToken(), if we fail to allocate space for
token_opaque_val, don't immediately pass the null pointer
to osi_Free on the error exit path.

Change-Id: Ic95e178cfbc1b1bbcb18701c0bbd5221426342ee
Found-by: clang static analyzer with help from AFS_NONNULL
Reviewed-on: http://gerrit.openafs.org/5262
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>