LINUX: you dont need to memset() after allocating credentials

LINUX: you dont need to memset() after allocating credentials

If you wanted to create a blank credential, you wouldn't want to make
it uid = 0.  Anyone allocating a credential SHOULD properly fill in all
fields making this blanking operation moot.  Regardless, this memset()
is before the allocation failure test and would/could panic.

Change-Id: Ia182a874a5e1bf28a2cd94898f67d81e5588d58c
Reviewed-on: http://gerrit.openafs.org/1080
Tested-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>