Fix warnings in vlserver/vlprocs.c

Fix warnings in vlserver/vlprocs.c

rxinfo's code to print out the principal corresponding to an rx_call uses
static arrays for the principal components but was checking that the array
pointer was non-NULL when deciding whether to print principal components.
Instead check whether each portion of the principal is the empty string.

Add explicit initializations of the static buffers to the empty string so
that we're not relying on rxkad_GetServerInfo always initializing them for
us.

Reviewed-on: http://gerrit.openafs.org/http://gerrit.openafs.org/163
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>