anonid: ensure string is NUL terminated

Fixes most recent symptom of b/31014535.

Change-Id: I88d934a63dc1860649d8c1db848ed2867734b2a1
diff --git a/cmds/anonid.c b/cmds/anonid.c
index e7854ad..106f5fc 100644
--- a/cmds/anonid.c
+++ b/cmds/anonid.c
@@ -177,6 +177,7 @@
     usage(argv[0]);
   }
 
+  memset(anonid, 0, sizeof(anonid));
   get_anonid_for_mac(addr, anonid);
   printf("%s\n", anonid);