[Minor] Add special codes to mark invalid request/reply

contrib/librdns/rdns.h

@@ -45,6 +45,7 @@ struct rdns_io_channel;
 typedef void (*dns_callback_type) (struct rdns_reply *reply, void *arg);
 
 enum rdns_request_type {
+	RDNS_REQUEST_INVALID = -1,
 	RDNS_REQUEST_A = 1,
 	RDNS_REQUEST_NS = 2,
 	RDNS_REQUEST_SOA = 6,
@@ -111,6 +112,7 @@ struct rdns_reply_entry {
 
 
 enum dns_rcode {
+	RDNS_RC_INVALID = -1,
 	RDNS_RC_NOERROR	= 0,
 	RDNS_RC_FORMERR	= 1,
 	RDNS_RC_SERVFAIL	= 2,

contrib/librdns/util.c

@@ -286,7 +286,7 @@ rdns_type_fromstr (const char *str)
 		}
 	}
 
-	return -1;
+	return RDNS_REQUEST_INVALID;
 }
 
 enum dns_rcode
@@ -334,7 +334,7 @@ rdns_rcode_fromstr (const char *str)
 		}
 	}
 
-	return -1;
+	return RDNS_RC_INVALID;
 }
 
 uint16_t