Fix getinfo_helper_networkstatus to return 0 on question-not-known

It erroneously returned -1 instead, which made it say "Internal error"
rather than "Unrecognized key."

src/or/networkstatus.c

@@ -2160,7 +2160,7 @@ getinfo_helper_networkstatus(control_connection_t *conn,
     *answer = networkstatus_getinfo_by_purpose(question+11, time(NULL));
     return *answer ? 0 : -1;
   } else {
-    return -1;
+    return 0;
   }
 
   if (status)