viced/callback.c: Ignore dump write errors even harder

viced/callback.c: Ignore dump write errors even harder

Not only do we need to check the return value of write(2), but
we also need to do so in a way that does not leave an empty body
in the if statement, in order to appease the clang-500.2.79 found
on OS X 10.9 with Xcode 5.0.2.

Change-Id: I4564f05927fe14fea3365e9e250834ee948fe387
Reviewed-on: http://gerrit.openafs.org/10697
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>