vtc.workspace_reserve() zero memory like vtc.workspace_alloc()

This is useful to ensure that workspace does not, by chance or
accident, contain data and, in particular, magic numbers from
previous workspace use.

vmod/vmod_vtc.c

@@ -194,6 +194,7 @@ vmod_workspace_reserve(VRT_CTX, VCL_ENUM which, VCL_INT size)
 	r = WS_ReserveSize(ws, size);
 	if (r == 0)
 		return (0);
+	memset(WS_Reservation(ws), 0, r);
 	WS_Release(ws, 0);
 	return (r);
 }

vmod/vmod_vtc.vcc

@@ -101,8 +101,8 @@ architecture. A failed allocation fails the transaction.
 $Function BYTES workspace_reserve(ENUM { client, backend, session, thread },
 	INT size)
 
-Attempt to reserve *size* bytes and release the reservation right
-away. Return the size of the reservation.
+Attempt to reserve *size* bytes, zero out that memory and release the
+reservation right away. Return the size of the reservation.
 
 See `vtc.workspace_alloc()`_ for semantics of the *size* argument.