bosserver: Don't store result when renaming core file

The bosserver doesn't do anything if the core file rename fails, so
don't bother storing the result code.

Caught by clang-analyzer

Change-Id: I9b1b7d77404b0fd560c7735646e5afe1fbcc0752
Reviewed-on: http://gerrit.openafs.org/9232
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Chas Williams - CONTRACTOR <chas@cmf.nrl.navy.mil>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>