Avoid salvager vol header read assert

Avoid salvager vol header read assert

When we read the volume header in order to write it back with a modified
inUse while salvaging, we were aborting if we couldn't read the header.
Since we can fail to read the header data if the volume header file
isn't associated with any data (and will be deleted by the salvager),
don't abort. Do still abort if we can't write the data back, since if
the data can be read but not written, other programs may think that the
volume is not being salvaged.

Reviewed-on: http://gerrit.openafs.org/480
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>