OPENAFS-SA-2018-002 butc: prevent TC_ReadLabel information leak

TC_ReadLabel (backup readlabel) does not initialize its output buffer
completely.  It leaks butc memory contents over the wire:

struct tc_tapeLabel
- up to 32 bytes from member afsname (TC_MAXTAPELEN 32)
- up to 32 bytes from member pname (TC_MAXTAPELEN 32)

Initialize the buffer.

[kaduk@mit.edu: move initialization to the RPC stub]

(cherry picked from commit 52f4d63148)

(cherry picked from commit b7e53b9e97)

(cherry picked from commit 3e0294543d)

Change-Id: I4e8ab1b94d36e9904a9505cd7f0e97cc6fb3a40f

src/butc/tcprocs.c

@@ -388,6 +388,7 @@ STC_ReadLabel(struct rx_call *acid, struct tc_tapeLabel *label, afs_uint32 *task
 {
     afs_int32 code;
 
+    memset(label, 0, sizeof(*label));
 #ifdef xbsa
     if (CONF_XBSA)
 	return (TC_BADTASK);	/* ReadLabel does not apply if XBSA */